183-431: Reality is the sum or aggregate of all that is real or existent within the universe , as opposed to that which is only imaginary , nonexistent or nonactual. The term is also used to refer to the ontological status of things, indicating their existence . In physical terms, reality is the totality of a system, known and unknown. Philosophical questions about the nature of reality or existence or being are considered under
366-558: A Hubble sphere . Some disputed estimates for the total size of the universe, if finite, reach as high as 10 10 10 122 {\displaystyle 10^{10^{10^{122}}}} megaparsecs, as implied by a suggested resolution of the No-Boundary Proposal . Assuming that the Lambda-CDM model is correct, the measurements of the parameters using a variety of techniques by numerous experiments yield
549-475: A god or gods exist, whether numbers and other abstract objects exist, and whether possible worlds exist. Epistemology is concerned with what can be known or inferred as likely and how, whereby in the modern world emphasis is put on reason , empirical evidence and science as sources and methods to determine or investigate reality. A common colloquial usage would have reality mean "perceptions, beliefs, and attitudes toward reality", as in "My reality
732-573: A TOE, it would necessarily be a set of equations. He wrote, "What is it that breathes fire into the equations and makes a universe for them to describe?" On a much broader and more subjective level, private experiences, curiosity, inquiry, and the selectivity involved in personal interpretation of events shapes reality as seen by one and only one person and hence is called phenomenological . While this form of reality might be common to others as well, it could at times also be so unique to oneself as to never be experienced or agreed upon by anyone else. Much of
915-421: A belief or we don't have a belief") with the more permissive, probabilistic notion of credence ("there is an entire spectrum of degrees of belief, not a simple dichotomy between belief and non-belief"). Philosophy addresses two different aspects of the topic of reality: the nature of reality itself, and the relationship between the mind (as well as language and culture) and reality. On the one hand, ontology
1098-462: A best value of the age of the universe at 13.799 ± 0.021 billion years, as of 2015. Over time, the universe and its contents have evolved. For example, the relative population of quasars and galaxies has changed and the universe has expanded . This expansion is inferred from the observation that the light from distant galaxies has been redshifted , which implies that the galaxies are receding from us. Analyses of Type Ia supernovae indicate that
1281-419: A certain amount. Constructivism and intuitionism are realistic about objects that can be explicitly constructed, but reject the use of the principle of the excluded middle to prove existence by reductio ad absurdum . The traditional debate has focused on whether an abstract (immaterial, intelligible) realm of numbers has existed in addition to the physical (sensible, concrete) world. A recent development
1464-484: A constantly operative efficient cause as long as it continues. This commitment appears most starkly to modern eyes in Aristotle’s discussion of projectile motion: what keeps the projectile moving after it leaves the hand? "Impetus", "momentum", much less "inertia", are not possible answers. There must be a mover, distinct (at least in some sense) from the thing moved, which is exercising its motive capacity at every moment of
1647-420: A critique of Aristotelian physics where he negated Aristotle's idea that a constant force produces uniform motion, as he realized that a force applied continuously produces acceleration , a fundamental law of classical mechanics and an early foreshadowing of Newton's second law of motion . Like Newton, he described acceleration as the rate of change of speed . In the 14th century, Jean Buridan developed
1830-415: A distinction between reality and existence. In fact, many analytic philosophers today tend to avoid the term "real" and "reality" in discussing ontological issues. But for those who would treat "is real" the same way they treat "exists", one of the leading questions of analytic philosophy has been whether existence (or reality) is a property of objects. It has been widely held by analytic philosophers that it
2013-696: A firm basis for all human knowledge , including scientific knowledge , and could establish philosophy as a "rigorous science". Husserl's conception of phenomenology has been criticised and developed by his student and assistant Martin Heidegger (1889–1976), by existentialists like Maurice Merleau-Ponty (1908–1961) and Jean-Paul Sartre (1905–1980), and by other philosophers, such as Paul Ricoeur (1913–2005), Emmanuel Levinas (1906–1995), and Dietrich von Hildebrand (1889–1977). Skeptical hypotheses in philosophy suggest that reality could be very different from what we think it is; or at least that we cannot prove it
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#17328525902672196-402: A galaxy have planets . At the largest scale , galaxies are distributed uniformly and the same in all directions, meaning that the universe has neither an edge nor a center. At smaller scales, galaxies are distributed in clusters and superclusters which form immense filaments and voids in space, creating a vast foam-like structure. Discoveries in the early 20th century have suggested that
2379-424: A hurled body also acquires an inclination (or "motive power") for movement away from whatever caused it to move, an inclination that secures its continued motion. This impressed virtue would be temporary and self-expending, meaning that all motion would tend toward the form of Aristotle's natural motion. In The Book of Healing (1027), the 11th-century Persian polymath Avicenna developed Philoponean theory into
2562-627: A man may exercise for the sake of his health: and so "health", and not just the hope of achieving it, is the cause of his action (this distinction is not trivial). But the eyelids are for the sake of the eye (to protect it: PA II.1 3) and the eye for the sake of the animal as a whole (to help it function properly: cf. An II.7). According to Aristotle, the science of living things proceeds by gathering observations about each natural kind of animal, organizing them into genera and species (the differentiae in History of Animals ) and then going on to study
2745-405: A number in the neighborhood of fifty spheres. An unmoved mover is assumed for each sphere, including a "prime mover" for the sphere of fixed stars . The unmoved movers do not push the spheres (nor could they, being immaterial and dimensionless) but are the final cause of the spheres' motion, i.e. they explain it in a way that's similar to the explanation "the soul is moved by beauty". Unlike
2928-399: A particular function ( Hilary Putnam ). Some have also attempted to offer significant revisions to our notion of belief, including eliminativists about belief who argue that there is no phenomenon in the natural world which corresponds to our folk psychological concept of belief ( Paul Churchland ) and formal epistemologists who aim to replace our bivalent notion of belief ("either we have
3111-454: A person who if asked about the color of snow would assert "snow is white"). There are various ways that contemporary philosophers have tried to describe beliefs, including as representations of ways that the world could be ( Jerry Fodor ), as dispositions to act as if certain things are true ( Roderick Chisholm ), as interpretive schemes for making sense of someone's actions ( Daniel Dennett and Donald Davidson ), or as mental states that fill
3294-455: A physically 'real' world". The hypothesis suggests that worlds corresponding to different sets of initial conditions, physical constants, or altogether different equations should be considered real. The theory can be considered a form of Platonism in that it posits the existence of mathematical entities, but can also be considered a mathematical monism in that it denies that anything exists except mathematical objects. The problem of universals
3477-539: A reasonably good account of various observations about the universe. The initial hot, dense state is called the Planck epoch , a brief period extending from time zero to one Planck time unit of approximately 10 seconds. During the Planck epoch, all types of matter and all types of energy were concentrated into a dense state, and gravity —currently the weakest by far of the four known forces —is believed to have been as strong as
3660-442: A set of four coordinates: ( x , y , z , t ) . On average, space is observed to be very nearly flat (with a curvature close to zero), meaning that Euclidean geometry is empirically true with high accuracy throughout most of the universe. Spacetime also appears to have a simply connected topology , in analogy with a sphere, at least on the length scale of the observable universe. However, present observations cannot exclude
3843-524: A single value be produced with certainty). A closely related term is counterfactual definiteness (CFD), used to refer to the claim that one can meaningfully speak of the definiteness of results of measurements that have not been performed (i.e. the ability to assume the existence of objects, and properties of objects, even when they have not been measured). Local realism is a significant feature of classical mechanics, of general relativity , and of classical electrodynamics ; but not quantum mechanics . In
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#17328525902674026-406: A special position. The founders of quantum mechanics debated the role of the observer, and of them, Wolfgang Pauli and Werner Heisenberg believed that quantum mechanics expressed the observers knowledge and when an experiment was completed the additional knowledge should be incorporated in the wave function, an effect that came to be called state reduction or collapse . This point of view, which
4209-428: A subconscious set of mental filters formed from their beliefs and experiences, every individual interprets the same world differently, hence "Truth is in the eye of the beholder". His ideas influenced the work of his friend Robert Anton Wilson . The status of abstract entities, particularly numbers, is a topic of discussion in mathematics. In the philosophy of mathematics, the best known form of realism about numbers
4392-533: A systematic framework we use to structure our experience. Spatial measurements are used to quantify how far apart objects are, and temporal measurements are used to quantitatively compare the interval between (or duration of) events . Although space and time are held to be transcendentally ideal in this sense, they are also empirically real , i.e. not mere illusions. Idealist writers such as J. M. E. McTaggart in The Unreality of Time have argued that time
4575-448: A tendency: in the way that glass objects tend to break, or are disposed to break, even if they do not actually break. Likewise, the mind-independent properties of quantum systems could consist of a tendency to respond to particular measurements with particular values with ascertainable probability. Such an ontology would be metaphysically realistic, without being realistic in the physicist's sense of "local realism" (which would require that
4758-465: A thing is that of which it is made. For a table, that might be wood; for a statue, that might be bronze or marble. "In one way we say that the aition is that out of which. as existing, something comes to be, like the bronze for the statue, the silver for the phial, and their genera" (194b2 3—6). By "genera", Aristotle means more general ways of classifying the matter (e.g. "metal"; "material"); and that will become important. A little later on, he broadens
4941-691: A work now called the EPR paradox , Einstein relied on local realism to suggest that hidden variables were missing in quantum mechanics. However, John S. Bell subsequently showed that the predictions of quantum mechanics are inconsistent with hidden variables, a result known as Bell's theorem . The predictions of quantum mechanics have been verified: Bell's inequalities are violated, meaning either local realism or counterfactual definiteness must be incorrect. Different interpretations of quantum mechanics violate different parts of local realism and/or counterfactual definiteness . The transition from "possible" to "actual"
5124-399: Is Platonic realism , which grants them abstract, immaterial existence. Other forms of realism identify mathematics with the concrete physical universe. Anti-realist stances include formalism and fictionalism . Some approaches are selectively realistic about some mathematical objects but not others. Finitism rejects infinite quantities. Ultra-finitism accepts finite quantities up to
5307-552: Is not a property at all, though this view has lost some ground in recent decades. On the other hand, particularly in discussions of objectivity that have feet in both metaphysics and epistemology, philosophical discussions of "reality" often concern the ways in which reality is, or is not, in some way dependent upon (or, to use fashionable jargon , "constructed" out of) mental and cultural factors such as perceptions, beliefs, and other mental states, as well as cultural artifacts, such as religions and political movements , on up to
5490-445: Is a composite particle made of quarks held together by the strong force . Hadrons are categorized into two families: baryons (such as protons and neutrons ) made of three quarks, and mesons (such as pions ) made of one quark and one antiquark . Of the hadrons, protons are stable, and neutrons bound within atomic nuclei are stable. Other hadrons are unstable under ordinary conditions and are thus insignificant constituents of
5673-401: Is a major topic of quantum physics , with related theories including quantum darwinism . The quantum mind –body problem refers to the philosophical discussions of the mind–body problem in the context of quantum mechanics. Since quantum mechanics involves quantum superpositions , which are not perceived by observers , some interpretations of quantum mechanics place conscious observers in
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5856-454: Is a perennial topic in metaphysics. For instance, Parmenides taught that reality was a single unchanging Being, whereas Heraclitus wrote that all things flow. The 20th-century philosopher Heidegger thought previous philosophers have lost sight of the question of Being (qua Being) in favour of the questions of beings (existing things), so he believed that a return to the Parmenidean approach
6039-454: Is a point to describing the world in such teleologically loaded terms: it makes sense of things in a way that atomist speculations do not. And further, Aristotle’s talk of species-forms is not as empty as his opponents would insinuate. He doesn't simply say that things do what they do because that's the sort of thing they do: the whole point of his classificatory biology, most clearly exemplified in PA ,
6222-408: Is a subjective attitude that a proposition is true or a state of affairs is the case. A subjective attitude is a mental state of having some stance , take, or opinion about something. In epistemology , philosophers use the term "belief" to refer to attitudes about the world which can be either true or false . To believe something is to take it to be true; for instance, to believe that snow
6405-531: Is a uniform circular motion at a given rate (relative to the diurnal motion of the outermost sphere of fixed stars). The concentric, aetherial, cheek-by-jowl " crystal spheres " that carry the Sun, Moon and stars move eternally with unchanging circular motion. Spheres are embedded within spheres to account for the "wandering stars" (i.e. the planets , which, in comparison with the Sun, Moon and stars, appear to move erratically). Mercury, Venus, Mars, Jupiter, and Saturn are
6588-557: Is about universal forms , in so far as they have been successfully understood, based on our memory of having encountered instances of those forms directly. Aristotle’s theory of cognition rests on two central pillars: his account of perception and his account of thought. Together, they make up a significant portion of his psychological writings, and his discussion of other mental states depends critically on them. These two activities, moreover, are conceived of in an analogous manner, at least with regard to their most basic forms. Each activity
6771-420: Is an ancient problem in metaphysics about whether universals exist. Universals are general or abstract qualities, characteristics, properties , kinds or relations , such as being male/female, solid/liquid/gas or a certain colour, that can be predicated of individuals or particulars or that individuals or particulars can be regarded as sharing or participating in. For example, Scott, Pat, and Chris have in common
6954-521: Is an illusion. As well as differing about the reality of time as a whole, metaphysical theories of time can differ in their ascriptions of reality to the past , present and future separately. Time, and the related concepts of process and evolution are central to the system-building metaphysics of A. N. Whitehead and Charles Hartshorne . The term " possible world " goes back to Leibniz's theory of possible worlds, used to analyse necessity, possibility , and similar modal notions . Modal realism
7137-417: Is called the observable universe . The proper distance (measured at a fixed time) between Earth and the edge of the observable universe is 46 billion light-years (14 billion parsecs ), making the diameter of the observable universe about 93 billion light-years (28 billion parsecs). Although the distance traveled by light from the edge of the observable universe is close to the age of the universe times
7320-484: Is closely related to essence and definition . He says for example that the ratio 2:1, and number in general, is the cause of the octave . "Another [cause] is the form and the exemplar: this is the formula (logos) of the essence (to ti en einai) , and its genera, for instance the ratio 2:1 of the octave" ( Phys 11.3 194b26—8)... Form is not just shape... We are asking (and this is the connection with essence, particularly in its canonical Aristotelian formulation) what it
7503-411: Is composed almost completely of dark energy, dark matter, and ordinary matter . Other contents are electromagnetic radiation (estimated to constitute from 0.005% to close to 0.01% of the total mass–energy of the universe) and antimatter . The proportions of all types of matter and energy have changed over the history of the universe. The total amount of electromagnetic radiation generated within
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7686-405: Is composed of two types of elementary particles : quarks and leptons . For example, the proton is formed of two up quarks and one down quark ; the neutron is formed of two down quarks and one up quark; and the electron is a kind of lepton. An atom consists of an atomic nucleus , made up of protons and neutrons (both of which are baryons ), and electrons that orbit the nucleus. Soon after
7869-646: Is equal to, less than, or greater than 1. These are called, respectively, the flat, open and closed universes. Observations, including the Cosmic Background Explorer (COBE), Wilkinson Microwave Anisotropy Probe (WMAP), and Planck maps of the CMB, suggest that the universe is infinite in extent with a finite age, as described by the Friedmann–Lemaître–Robertson–Walker (FLRW) models. These FLRW models thus support inflationary models and
8052-519: Is everywhere the cause of order. While consistent with common human experience, Aristotle's principles were not based on controlled, quantitative experiments, so they do not describe our universe in the precise, quantitative way now expected of science. Contemporaries of Aristotle like Aristarchus rejected these principles in favor of heliocentrism , but their ideas were not widely accepted. Aristotle's principles were difficult to disprove merely through casual everyday observation, but later development of
8235-927: Is found in atoms and is directly tied to all chemical properties . Neutrinos rarely interact with anything, and are consequently rarely observed. Neutrinos stream throughout the universe but rarely interact with normal matter. Aristotelian physics Aristotelian physics is the form of natural philosophy described in the works of the Greek philosopher Aristotle (384–322 BC). In his work Physics , Aristotle intended to establish general principles of change that govern all natural bodies, both living and inanimate, celestial and terrestrial – including all motion (change with respect to place), quantitative change (change with respect to size or number), qualitative change, and substantial change (" coming to be " [coming into existence , 'generation'] or "passing away" [no longer existing, 'corruption']). To Aristotle, 'physics'
8418-410: Is generated and destroyed. Will this be, or not? Yes, if this happens; otherwise not ( Metaphysics VI, 1027a29). Aristotle argues against the indivisibles of Democritus (which differ considerably from the historical and the modern use of the term " atom "). As a place without anything existing at or within it, Aristotle argued against the possibility of a vacuum or void. Because he believed that
8601-597: Is in the Local Group of galaxies, which in turn is in the Laniakea Supercluster . This supercluster spans over 500 million light-years, while the Local Group spans over 10 million light-years. The universe also has vast regions of relative emptiness; the largest known void measures 1.8 billion ly (550 Mpc) across. The observable universe is isotropic on scales significantly larger than superclusters, meaning that
8784-523: Is known as dark matter . In the widely accepted ΛCDM cosmological model, dark matter accounts for about 25.8% ± 1.1% of the mass and energy in the universe while about 69.2% ± 1.2% is dark energy , a mysterious form of energy responsible for the acceleration of the expansion of the universe . Ordinary (' baryonic ') matter therefore composes only 4.84% ± 0.1% of the universe. Stars, planets, and visible gas clouds only form about 6% of this ordinary matter. There are many competing hypotheses about
8967-456: Is known as direct realism when developed to counter indirect or representative realism, also known as epistemological dualism , the philosophical position that our conscious experience is not of the real world itself but of an internal representation, a miniature virtual-reality replica of the world. Timothy Leary coined the influential term Reality Tunnel , by which he means a kind of representative realism . The theory states that, with
9150-466: Is likewise a concentric shell surrounding that of water; bubbles rise in water. Finally, the natural place of fire is higher than that of air but below the innermost celestial sphere (carrying the Moon). In Book Delta of his Physics (IV.5), Aristotle defines topos (place) in terms of two bodies, one of which contains the other: a "place" is where the inner surface of the former (the containing body) touches
9333-523: Is no mind or soul over and above such mental events . Finally, anti-realism became a fashionable term for any view which held that the existence of some object depends upon the mind or cultural artifacts. The view that the so-called external world is really merely a social, or cultural, artifact, called social constructionism , is one variety of anti-realism. Cultural relativism is the view that social issues such as morality are not absolute, but at least partially cultural artifact . The nature of being
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#17328525902679516-470: Is not your reality." This is often used just as a colloquialism indicating that the parties to a conversation agree, or should agree, not to quibble over deeply different conceptions of what is real. For example, in a religious discussion between friends, one might say (attempting humor), "You might disagree, but in my reality, everyone goes to heaven." Reality can be defined in a way that links it to worldviews or parts of them (conceptual frameworks): Reality
9699-403: Is not. Examples include: Jain philosophy postulates that seven tattva (truths or fundamental principles) constitute reality. These seven tattva are: Scientific realism is, at the most general level, the view that the world (the universe) described by science (perhaps ideal science) is the real world, as it is, independent of what we might take it to be. Within philosophy of science , it
9882-557: Is often framed as an answer to the question "how is the success of science to be explained?" The debate over what the success of science involves centers primarily on the status of entities that are not directly observable discussed by scientific theories . Generally, those who are scientific realists state that one can make reliable claims about these entities (viz., that they have the same ontological status) as directly observable entities, as opposed to instrumentalism . The most used and studied scientific theories today state more or less
10065-498: Is related to alethic logic : a proposition is necessary if it is true in all possible worlds, and possible if it is true in at least one. The many worlds interpretation of quantum mechanics is a similar idea in science. The philosophical implications of a physical TOE are frequently debated. For example, if philosophical physicalism is true, a physical TOE will coincide with a philosophical theory of everything. The "system building" style of metaphysics attempts to answer all
10248-405: Is said not to depend on perceptions, beliefs, language, or any other human artifact, one can speak of "realism about " that object. A correspondence theory of knowledge about what exists claims that "true" knowledge of reality represents accurate correspondence of statements about and images of reality with the actual reality that the statements or images are attempting to represent. For example,
10431-782: Is subject to the Pauli exclusion principle ; no two leptons of the same species can be in exactly the same state at the same time. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos ). Electrons are stable and the most common charged lepton in the universe, whereas muons and taus are unstable particles that quickly decay after being produced in high energy collisions, such as those involving cosmic rays or carried out in particle accelerators . Charged leptons can combine with other particles to form various composite particles such as atoms and positronium . The electron governs nearly all of chemistry , as it
10614-447: Is that there simply and literally is no reality beyond the perceptions or beliefs we each have about reality. Such attitudes are summarized in popular statements, such as "Perception is reality" or "Life is how you perceive reality" or "reality is what you can get away with" ( Robert Anton Wilson ), and they indicate anti-realism – that is, the view that there is no objective reality, whether acknowledged explicitly or not. Many of
10797-480: Is the Standard Model , a theory that is concerned with electromagnetic interactions and the weak and strong nuclear interactions. The Standard Model is supported by the experimental confirmation of the existence of particles that compose matter: quarks and leptons , and their corresponding " antimatter " duals, as well as the force particles that mediate interactions : the photon , the W and Z bosons , and
10980-525: Is the mathematical universe hypothesis , the theory that only a mathematical world exists, with the finite, physical world being an illusion within it. An extreme form of realism about mathematics is the mathematical multiverse hypothesis advanced by Max Tegmark . Tegmark's sole postulate is: All structures that exist mathematically also exist physically . That is, in the sense that "in those [worlds] complex enough to contain self-aware substructures [they] will subjectively perceive themselves as existing in
11163-607: Is the adult plant, for a ball at the top of a ramp, it is coming to rest at the bottom, for an eye, it is seeing, for a knife, it is cutting. Goals have an explanatory function: that is a commonplace, at least in the context of action-ascriptions. Less of a commonplace is the view espoused by Aristotle, that finality and purpose are to be found throughout nature, which is for him the realm of those things which contain within themselves principles of movement and rest (i.e. efficient causes); thus it makes sense to attribute purposes not only to natural things themselves, but also to their parts:
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#173285259026711346-441: Is the definition of the animal’s substantial being ( GA I.1 715a4: ho logos tês ousias ). The final cause is the adult form, which is the end for the sake of which development takes place. The four elements make up the uniform materials such as blood, flesh and bone, which are themselves the matter out of which are created the non-uniform organs of the body (e.g. the heart, liver and hands) "which in turn, as parts, are matter for
11529-405: Is the study of being, and the central topic of the field is couched, variously, in terms of being, existence, "what is", and reality. The task in ontology is to describe the most general categories of reality and how they are interrelated. If a philosopher wanted to proffer a positive definition of the concept "reality", it would be done under this heading. As explained above, some philosophers draw
11712-404: Is the totality of all things, structures (actual and conceptual), events (past and present) and phenomena, whether observable or not. It is what a world view (whether it be based on individual or shared human experience) ultimately attempts to describe or map. Certain ideas from physics, philosophy, sociology, literary criticism , and other fields shape various theories of reality. One such theory
11895-488: Is the view, notably propounded by David Kellogg Lewis , that all possible worlds are as real as the actual world. In short: the actual world is regarded as merely one among an infinite set of logically possible worlds, some "nearer" to the actual world and some more remote. Other theorists may use the Possible World framework to express and explore problems without committing to it ontologically. Possible world theory
12078-460: Is there any definite cause for an accident, but only chance (τύχη, týche ), namely an indefinite (ἀόριστον, aóriston ) cause" ( Metaphysics V, 1025a25). It is obvious that there are principles and causes which are generable and destructible apart from the actual processes of generation and destruction; for if this is not true, everything will be of necessity: that is, if there must necessarily be some cause, other than accidental, of that which
12261-520: Is to be some thing. And it is a feature of musical harmonics (first noted and wondered at by the Pythagoreans) that intervals of this type do indeed exhibit this ratio in some form in the instruments used to create them (the length of pipes, of strings, etc.). In some sense, the ratio explains what all the intervals have in common, why they turn out the same. The efficient cause of a thing is the primary agency by which its matter took its form. For example,
12444-620: Is to show what sorts of function go with what, which presuppose which and which are subservient to which. And in this sense, formal or functional biology is susceptible of a type of reductionism. We start, he tells us, with the basic animal kinds which we all pre-theoretically (although not indefeasibly) recognize (cf. PA I.4): but we then go on to show how their parts relate to one another: why it is, for instance, that only blooded creatures have lungs, and how certain structures in one species are analogous or homologous to those in another (such as scales in fish, feathers in birds, hair in mammals). And
12627-492: Is triggered by its object – each, that is, is about the very thing that brings it about. This simple causal account explains the reliability of cognition: perception and thought are, in effect, transducers, bringing information about the world into our cognitive systems, because, at least in their most basic forms, they are infallibly about the causes that bring them about ( An III.4 429a13–18). Other, more complex mental states are far from infallible. But they are still tethered to
12810-430: Is unknown whether or not they are composed of smaller and even more fundamental particles. In most contemporary models they are thought of as points in space. All elementary particles are currently best explained by quantum mechanics and exhibit wave–particle duality : their behavior has both particle-like and wave -like aspects, with different features dominating under different circumstances. Of central importance
12993-869: Is unknown. Dark matter, a mysterious form of matter that has not yet been identified, accounts for 26.8% of the cosmic contents. Dark energy, which is the energy of empty space and is causing the expansion of the universe to accelerate, accounts for the remaining 68.3% of the contents. Matter, dark matter, and dark energy are distributed homogeneously throughout the universe over length scales longer than 300 million light-years (ly) or so. However, over shorter length-scales, matter tends to clump hierarchically; many atoms are condensed into stars , most stars into galaxies, most galaxies into clusters, superclusters and, finally, large-scale galactic filaments . The observable universe contains as many as an estimated 2 trillion galaxies and, overall, as many as an estimated 10 stars – more stars (and earth-like planets) than all
13176-405: Is white is comparable to accepting the truth of the proposition "snow is white". However, holding a belief does not require active introspection . For example, few individuals carefully consider whether or not the sun will rise tomorrow, simply assuming that it will. Moreover, beliefs need not be occurrent (e.g. a person actively thinking "snow is white"), but can instead be dispositional (e.g.
13359-415: Is won at the expense of any serious empirical content. Mechanism, at least as practiced by Aristotle’s contemporaries and predecessors, may have been explanatorily inadequate – but at least it was an attempt at a general account given in reductive terms of the lawlike connections between things. Simply introducing what later reductionists were to scoff at as "occult qualities" does not explain – it merely, in
13542-576: The Big Bang , primordial protons and neutrons formed from the quark–gluon plasma of the early universe as it cooled below two trillion degrees. A few minutes later, in a process known as Big Bang nucleosynthesis , nuclei formed from the primordial protons and neutrons. This nucleosynthesis formed lighter elements, those with small atomic numbers up to lithium and beryllium , but the abundance of heavier elements dropped off sharply with increasing atomic number. Some boron may have been formed at this time, but
13725-418: The Big Bang , would have completely annihilated each other and left only photons as a result of their interaction. These laws are Gauss's law and the non-divergence of the stress–energy–momentum pseudotensor . Due to the finite speed of light , there is a limit (known as the particle horizon ) to how far light can travel over the age of the universe . The spatial region from which we can receive light
13908-532: The German words Das All , Weltall , and Natur for universe . The same synonyms are found in English, such as everything (as in the theory of everything ), the cosmos (as in cosmology ), the world (as in the many-worlds interpretation ), and nature (as in natural laws or natural philosophy ). The prevailing model for the evolution of the universe is the Big Bang theory. The Big Bang model states that
14091-537: The Greek phainómenon , meaning "that which appears", and lógos , meaning "study". In Husserl's conception, phenomenology is primarily concerned with making the structures of consciousness, and the phenomena which appear in acts of consciousness, objects of systematic reflection and analysis. Such reflection was to take place from a highly modified " first person " viewpoint, studying phenomena not as they appear to "my" consciousness, but to any consciousness whatsoever. Husserl believed that phenomenology could thus provide
14274-509: The Local Void adjacent to our own galaxy ) have the opposite effect: ultimately, bodies off-center are ejected from the void due to the gravity of the material outside. According to Aristotle, there are four ways to explain the aitia or causes of change. He writes that "we do not have knowledge of a thing until we have grasped its why, that is to say, its cause." Aristotle held that there were four kinds of causes. The material cause of
14457-617: The Milky Way is roughly 100,000–180,000 light-years in diameter, and the nearest sister galaxy to the Milky Way, the Andromeda Galaxy , is located roughly 2.5 million light-years away. Because humans cannot observe space beyond the edge of the observable universe, it is unknown whether the size of the universe in its totality is finite or infinite. Estimates suggest that the whole universe, if finite, must be more than 250 times larger than
14640-798: The Physics is largely concerned with an analysis of motion, particularly local motion, and the other concepts that Aristotle believes are requisite to that analysis. There are clear differences between modern and Aristotelian physics, the main being the use of mathematics , largely absent in Aristotle. Some recent studies, however, have re-evaluated Aristotle's physics, stressing both its empirical validity and its continuity with modern physics. Aristotle divided his universe into "terrestrial spheres" which were "corruptible" and where humans lived, and moving but otherwise unchanging celestial spheres . Aristotle believed that four classical elements make up everything in
14823-458: The absolute a positive one. The question of direct or "naïve" realism , as opposed to indirect or "representational" realism , arises in the philosophy of perception and of mind out of the debate over the nature of conscious experience ; the epistemological question of whether the world we see around us is the real world itself or merely an internal perceptual copy of that world generated by neural processes in our brain. Naïve realism
15006-463: The expansion is accelerating . The more matter there is in the universe, the stronger the mutual gravitational pull of the matter. If the universe were too dense then it would re-collapse into a gravitational singularity . However, if the universe contained too little matter then the self-gravity would be too weak for astronomical structures, like galaxies or planets, to form. Since the Big Bang,
15189-404: The general theory of relativity , explains gravity by recognizing that spacetime is not fixed but instead dynamical. In general relativity, gravitational force is reimagined as curvature of spacetime . A curved path like an orbit is not the result of a force deflecting a body from an ideal straight-line path, but rather the body's attempt to fall freely through a background that is itself curved by
15372-553: The gluon . The Standard Model predicted the existence of the recently discovered Higgs boson , a particle that is a manifestation of a field within the universe that can endow particles with mass. Because of its success in explaining a wide variety of experimental results, the Standard Model is sometimes regarded as a "theory of almost everything". The Standard Model does not, however, accommodate gravity. A true force–particle "theory of everything" has not been attained. A hadron
15555-483: The grains of beach sand on planet Earth ; but less than the total number of atoms estimated in the universe as 10 ; and the estimated total number of stars in an inflationary universe (observed and unobserved), as 10 . Typical galaxies range from dwarfs with as few as ten million (10 ) stars up to giants with one trillion (10 ) stars. Between the larger structures are voids , which are typically 10–150 Mpc (33 million–490 million ly) in diameter. The Milky Way
15738-441: The hadron epoch , and the lepton epoch . Together, these epochs encompassed less than 10 seconds of time following the Big Bang. These elementary particles associated stably into ever larger combinations, including stable protons and neutrons , which then formed more complex atomic nuclei through nuclear fusion . This process, known as Big Bang nucleosynthesis , lasted for about 17 minutes and ended about 20 minutes after
15921-404: The large-scale structure of the universe. Other than neutrinos , a form of hot dark matter , dark matter has not been detected directly, making it one of the greatest mysteries in modern astrophysics . Dark matter neither emits nor absorbs light or any other electromagnetic radiation at any significant level. Dark matter is estimated to constitute 26.8% of the total mass–energy and 84.5% of
16104-399: The law of universal gravitation , Isaac Newton built upon Copernicus's work as well as Johannes Kepler 's laws of planetary motion and observations by Tycho Brahe . Further observational improvements led to the realization that the Sun is one of a few hundred billion stars in the Milky Way , which is one of a few hundred billion galaxies in the observable universe. Many of the stars in
16287-421: The observable universe and global geometry . Cosmologists often work with a given space-like slice of spacetime called the comoving coordinates . The section of spacetime which can be observed is the backward light cone , which delimits the cosmological horizon . The cosmological horizon, also called the particle horizon or the light horizon, is the maximum distance from which particles can have traveled to
16470-442: The observer in the age of the universe . This horizon represents the boundary between the observable and the unobservable regions of the universe. An important parameter determining the future evolution of the universe theory is the density parameter , Omega (Ω), defined as the average matter density of the universe divided by a critical value of that density. This selects one of three possible geometries depending on whether Ω
16653-573: The rubric of ontology , which is a major branch of metaphysics in the Western philosophical tradition. Ontological questions also feature in diverse branches of philosophy , including the philosophy of science , of religion , of mathematics , and philosophical logic . These include questions about whether only physical objects are real (i.e., physicalism ), whether reality is fundamentally immaterial (e.g. idealism ), whether hypothetical unobservable entities posited by scientific theories exist, whether
16836-503: The scientific method can verify that a statement is true based on the observable evidence that a thing exists. Many humans can point to the Rocky Mountains and say that this mountain range exists, and continues to exist even if no one is observing it or making statements about it. One can also speak of anti -realism about the same objects. Anti-realism is the latest in a long series of terms for views opposed to realism. Perhaps
17019-416: The scientific method challenged his views with experiments and careful measurement, using increasingly advanced technology such as the telescope and vacuum pump . In claiming novelty for their doctrines, those natural philosophers who developed the "new science" of the seventeenth century frequently contrasted "Aristotelian" physics with their own. Physics of the former sort, so they claimed, emphasized
17202-422: The sublunary orb's changeable, terrestrial matter, dragging the rarefied fire and air along underneath as it rotates. Like Homer 's æthere (αἰθήρ) – the "pure air" of Mount Olympus – was the divine counterpart of the air breathed by mortal beings (άήρ, aer ). The celestial spheres are composed of the special element aether , eternal and unchanging, the sole capability of which
17385-458: The theory of impetus as an alternative to the Aristotelian theory of motion. The theory of impetus was a precursor to the concepts of inertia and momentum in classical mechanics. Buridan and Albert of Saxony also refer to Abu'l-Barakat in explaining that the acceleration of a falling body is a result of its increasing impetus. In the 16th century, Al-Birjandi discussed the possibility of
17568-624: The ultimate fate of the universe and about what, if anything, preceded the Big Bang, while other physicists and philosophers refuse to speculate, doubting that information about prior states will ever be accessible. Some physicists have suggested various multiverse hypotheses, in which the universe might be one among many. The physical universe is defined as all of space and time (collectively referred to as spacetime ) and their contents. Such contents comprise all of energy in its various forms, including electromagnetic radiation and matter , and therefore planets, moons , stars, galaxies, and
17751-477: The weak and strong nuclear forces , decline very rapidly with distance; their effects are confined mainly to sub-atomic length scales. The universe appears to have much more matter than antimatter , an asymmetry possibly related to the CP violation . This imbalance between matter and antimatter is partially responsible for the existence of all matter existing today, since matter and antimatter, if equally produced at
17934-414: The 20th century, views similar to Berkeley's were called phenomenalism . Phenomenalism differs from Berkeleyan idealism primarily in that Berkeley believed that minds, or souls, are not merely ideas nor made up of ideas, whereas varieties of phenomenalism, such as that advocated by Russell , tended to go farther to say that the mind itself is merely a collection of perceptions, memories, etc., and that there
18117-421: The Big Bang, so only the fastest and simplest reactions occurred. About 25% of the protons and all the neutrons in the universe, by mass, were converted to helium , with small amounts of deuterium (a form of hydrogen ) and traces of lithium . Any other element was only formed in very tiny quantities. The other 75% of the protons remained unaffected, as hydrogen nuclei. After nucleosynthesis ended,
18300-409: The Sun, Moon, planets and stars – are embedded in perfectly concentric " crystal spheres " that rotate eternally at fixed rates. Because the celestial spheres are incapable of any change except rotation, the terrestrial sphere of fire must account for the heat, starlight and occasional meteorites . The lowest, lunar sphere is the only celestial sphere that actually comes in contact with
18483-433: The answers, for Aristotle, are to be found in the economy of functions, and how they all contribute to the overall well-being (the final cause in this sense) of the animal. According to Aristotle, perception and thought are similar, though not exactly alike in that perception is concerned only with the external objects that are acting on our sense organs at any given time, whereas we can think about anything we choose. Thought
18666-654: The cause ceases, so does the effect. The cause, according to Aristotle, must be a power (i.e., force) that drives the body as long as the external agent remains in direct contact. Aristotle went on to say that the velocity of the body is directly proportional to the force imparted and inversely proportional to the resistance of the medium in which the motion takes place. This gives the law in today's notation velocity ∝ imparted power resistance {\displaystyle {\text{velocity}}\propto {\frac {\text{imparted power}}{\text{resistance}}}} This law presented three difficulties that Aristotle
18849-474: The causes (in Parts of Animals and Generation of Animals , his three main biological works). The four causes of animal generation can be summarized as follows. The mother and father represent the material and efficient causes, respectively. The mother provides the matter out of which the embryo is formed, while the father provides the agency that informs that material and triggers its development. The formal cause
19032-400: The changer of the thing changed". Aristotle’s examples here are instructive: one case of mental and one of physical causation, followed by a perfectly general characterization. But they conceal (or at any rate fail to make patent) a crucial feature of Aristotle’s concept of efficient causation, and one which serves to distinguish it from most modern homonyms. For Aristotle, any process requires
19215-485: The comprehension of reality. Out of all the realities, the reality of everyday life is the most important one since our consciousness requires us to be completely aware and attentive to the experience of everyday life. In philosophy , potentiality and actuality are a pair of closely connected principles which Aristotle used to analyze motion , causality , ethics , and physiology in his Physics , Metaphysics , Nicomachean Ethics , and De Anima . A belief
19398-400: The concepts of science and philosophy are often defined culturally and socially . This idea was elaborated by Thomas Kuhn in his book The Structure of Scientific Revolutions (1962). The Social Construction of Reality , a book about the sociology of knowledge written by Peter L. Berger and Thomas Luckmann , was published in 1966. It explained how knowledge is acquired and used for
19581-425: The contained body. This definition remained dominant until the beginning of the 17th century, even though it had been questioned and debated by philosophers since antiquity. The most significant early critique was made in terms of geometry by the 11th-century Arab polymath al-Hasan Ibn al-Haytham ( Alhazen ) in his Discourse on Place . Terrestrial objects rise or fall, to a greater or lesser extent, according to
19764-476: The contents of intergalactic space . The universe also includes the physical laws that influence energy and matter, such as conservation laws , classical mechanics , and relativity . The universe is often defined as "the totality of existence", or everything that exists, everything that has existed, and everything that will exist. In fact, some philosophers and scientists support the inclusion of ideas and abstract concepts—such as mathematics and logic—in
19947-500: The definition of the universe. The word universe may also refer to concepts such as the cosmos , the world , and nature . The word universe derives from the Old French word univers , which in turn derives from the Latin word universus , meaning 'combined into one'. The Latin word 'universum' was used by Cicero and later Latin authors in many of the same senses as
20130-546: The density of matter was less than the density of dark energy, marking the beginning of the present dark-energy-dominated era . In this era, the expansion of the universe is accelerating due to dark energy. Of the four fundamental interactions , gravitation is the dominant at astronomical length scales. Gravity's effects are cumulative; by contrast, the effects of positive and negative charges tend to cancel one another, making electromagnetism relatively insignificant on astronomical length scales. The remaining two interactions,
20313-407: The earliest state of the universe was an extremely hot and dense one, and that the universe subsequently expanded and cooled. The model is based on general relativity and on simplifying assumptions such as the homogeneity and isotropy of space. A version of the model with a cosmological constant (Lambda) and cold dark matter , known as the Lambda-CDM model , is the simplest model that provides
20496-490: The early modern period include the Leibniz 's Monadology , Descartes 's Dualism , Spinoza 's Monism . Hegel 's Absolute idealism and Whitehead 's Process philosophy were later systems. Other philosophers do not believe its techniques can aim so high. Some scientists think a more mathematical approach than philosophy is needed for a TOE, for instance Stephen Hawking wrote in A Brief History of Time that even if we had
20679-412: The efficient cause of a baby is a parent of the same species and that of a table is a carpenter, who knows the form of the table. In his Physics II, 194b29—32, Aristotle writes: "there is that which is the primary originator of the change and of its cessation, such as the deliberator who is responsible [sc. for the action] and the father of the child, and in general the producer of the thing produced and
20862-411: The energy of each photon decreases as it is cosmologically redshifted . At around 47,000 years, the energy density of matter became larger than that of photons and neutrinos , and began to dominate the large scale behavior of the universe. This marked the end of the radiation-dominated era and the start of the matter-dominated era . In the earliest stages of the universe, tiny fluctuations within
21045-419: The eternal and unchanging celestial aether , each of the four terrestrial elements are capable of changing into either of the two elements they share a property with: e.g. the cold and wet ( water ) can transform into the hot and wet ( air ) or the cold and dry ( earth ). Any apparent change from cold and wet into the hot and dry ( fire ) is actually a two-step process, as first one of the property changes, then
21228-399: The first 10 seconds. This initial period of inflation would explain why space appears to be very flat . Within the first fraction of a second of the universe's existence, the four fundamental forces had separated. As the universe continued to cool from its inconceivably hot state, various types of subatomic particles were able to form in short periods of time known as the quark epoch ,
21411-476: The first subatomic particles and simple atoms to form. Giant clouds of hydrogen and helium were gradually drawn to the places where matter was most dense , forming the first galaxies, stars, and everything else seen today. From studying the effects of gravity on both matter and light, it has been discovered that the universe contains much more matter than is accounted for by visible objects; stars, galaxies, nebulas and interstellar gas. This unseen matter
21594-494: The first coherent alternative to Aristotelian theory. Inclinations in the Avicennan theory of motion were not self-consuming but permanent forces whose effects were dissipated only as a result of external agents such as air resistance, making him "the first to conceive such a permanent type of impressed virtue for non-natural motion". Such a self-motion ( mayl ) is "almost the opposite of the Aristotelian conception of violent motion of
21777-430: The first time. Unlike plasma, neutral atoms are transparent to many wavelengths of light, so for the first time the universe also became transparent. The photons released (" decoupled ") when these atoms formed can still be seen today; they form the cosmic microwave background (CMB). As the universe expands, the energy density of electromagnetic radiation decreases more quickly than does that of matter because
21960-561: The first was idealism , so called because reality was said to be in the mind, or a product of our ideas . Berkeleyan idealism is the view, propounded by the Irish empiricist George Berkeley , that the objects of perception are actually ideas in the mind. In this view, one might be tempted to say that reality is a "mental construct"; this is not quite accurate, however, since, in Berkeley's view, perceptual ideas are created and coordinated by God. By
22143-443: The following solution to the third problem in the case of a shot arrow. The bowstring or hand imparts a certain 'power of being a movent' to the air in contact with it, so that this imparted force is transmitted to the next layer of air, and so on, thus keeping the arrow in motion until the power gradually dissipates. In his Physics Aristotle examines accidents (συμβεβηκός, symbebekòs ) that have no cause but chance. "Nor
22326-439: The functioning body as a whole ( PA II. 1 646a 13—24)". [There] is a certain obvious conceptual economy about the view that in natural processes naturally constituted things simply seek to realize in full actuality the potentials contained within them (indeed, this is what is for them to be natural); on the other hand, as the detractors of Aristotelianism from the seventeenth century on were not slow to point out, this economy
22509-427: The gradual reionization of the universe between about 200–500 million years and 1 billion years, and also for seeding the universe with elements heavier than helium, through stellar nucleosynthesis . The universe also contains a mysterious energy—possibly a scalar field —called dark energy , the density of which does not change over time. After about 9.8 billion years, the universe had expanded sufficiently so that
22692-449: The ground in similar times. Apart from the natural tendency of terrestrial exhalations to rise and objects to fall , unnatural or forced motion from side to side results from the turbulent collision and sliding of the objects as well as transmutation between the elements ( On Generation and Corruption ). Aristotle phrased this principle as: "Everything that moves is moved by something else. (Omne quod moventur ab alio movetur.)" When
22875-427: The heavenly bodies "were accountable to the laws of physics ". During his debate with Avicenna , al-Biruni also criticized the Aristotelian theory of gravity firstly for denying the existence of levity or gravity in the celestial spheres ; and, secondly, for its notion of circular motion being an innate property of the heavenly bodies . Hibat Allah Abu'l-Barakat al-Baghdaadi (1080–1165) wrote al-Mu'tabar ,
23058-424: The important questions in a coherent way, providing a complete picture of the world. Plato and Aristotle could be said to be early examples of comprehensive systems. In the early modern period (17th and 18th centuries), the system-building scope of philosophy is often linked to the rationalist method of philosophy, that is the technique of deducing the nature of the world by pure a priori reason. Examples from
23241-647: The kind of experience deemed spiritual occurs on this level of reality. Phenomenology is a philosophical method developed in the early years of the twentieth century by Edmund Husserl (1859–1938) and a circle of followers at the universities of Göttingen and Munich in Germany. Subsequently, phenomenological themes were taken up by philosophers in France, the United States, and elsewhere, often in contexts far removed from Husserl's work. The word phenomenology comes from
23424-429: The manner of Molière’s famous satirical joke, serves to re-describe the effect. Formal talk, or so it is said, is vacuous. Things are not however quite as bleak as this. For one thing, there’s no point in trying to engage in reductionist science if you don’t have the wherewithal, empirical and conceptual, to do so successfully: science shouldn't be simply unsubstantiated speculative metaphysics. But more than that, there
23607-503: The material, formal, efficient, and final causes of things. As regards living things, Aristotle's biology relied on observation of what he considered to be ‘natural kinds’, both those he considered basic and the groups to which he considered these belonged. He did not conduct experiments in the modern sense, but relied on amassing data, observational procedures such as dissection , and making hypotheses about relationships between measurable quantities such as body size and lifespan. nature
23790-553: The mind. Some anti-realists whose ontological position is that objects outside the mind do exist, nevertheless doubt the independent existence of time and space. Kant , in the Critique of Pure Reason , described time as an a priori notion that, together with other a priori notions such as space , allows us to comprehend sense experience . Kant denies that either space or time are substance , entities in themselves, or learned by experience; he holds rather that both are elements of
23973-544: The modern English word is used. A term for universe among the ancient Greek philosophers from Pythagoras onwards was τὸ πᾶν ( tò pân ) 'the all', defined as all matter and all space, and τὸ ὅλον ( tò hólon ) 'all things', which did not necessarily include the void. Another synonym was ὁ κόσμος ( ho kósmos ) meaning 'the world , the cosmos '. Synonyms are also found in Latin authors ( totum , mundus , natura ) and survive in modern languages, e.g.,
24156-452: The modern sense of the word). Instead, they are abstractions used to explain the varying natures and behaviors of actual materials in terms of ratios between them. Motion and change are closely related in Aristotelian physics. Motion, according to Aristotle, involved a change from potentiality to actuality . He gave example of four types of change, namely change in substance, in quality, in quantity and in place. Aristotle proposed that
24339-448: The modern universe. From approximately 10 seconds after the Big Bang , during a period known as the hadron epoch , the temperature of the universe had fallen sufficiently to allow quarks to bind together into hadrons, and the mass of the universe was dominated by hadrons . Initially, the temperature was high enough to allow the formation of hadron–anti-hadron pairs, which kept matter and antimatter in thermal equilibrium . However, as
24522-541: The next heavier element, carbon , was not formed in significant amounts. Big Bang nucleosynthesis shut down after about 20 minutes due to the rapid drop in temperature and density of the expanding universe. Subsequent formation of heavier elements resulted from stellar nucleosynthesis and supernova nucleosynthesis . Ordinary matter and the forces that act on matter can be described in terms of elementary particles . These particles are sometimes described as being fundamental, since they have an unknown substructure, and it
24705-518: The observed rate of expansion. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to vacuum energy . Dark matter is a hypothetical kind of matter that is invisible to the entire electromagnetic spectrum , but which accounts for most of the matter in the universe. The existence and properties of dark matter are inferred from its gravitational effects on visible matter, radiation, and
24888-413: The only planets (including minor planets ) which were visible before the invention of the telescope, which is why Neptune and Uranus are not included, nor are any asteroids . Later, the belief that all spheres are concentric was forsaken in favor of Ptolemy 's deferent and epicycle model. Aristotle submits to the calculations of astronomers regarding the total number of spheres and various accounts give
25071-428: The other fundamental forces, and all the forces may have been unified . The physics controlling this very early period (including quantum gravity in the Planck epoch) is not understood, so we cannot say what, if anything, happened before time zero . Since the Planck epoch, the universe has been expanding to its present scale, with a very short but intense period of cosmic inflation speculated to have occurred within
25254-399: The other hand, is the view that universals are real entities, but their existence is dependent on the particulars that exemplify them. Nominalism and conceptualism are the main forms of anti-realism about universals. A traditional realist position in ontology is that time and space have existence apart from the human mind. Idealists deny or doubt the existence of objects independent of
25437-520: The other three terrestrial elements. Other, lighter objects, he believed, have less earth, relative to the other three elements in their composition. The four classical elements were not invented by Aristotle; they were originated by Empedocles . During the Scientific Revolution , the ancient theory of classical elements was found to be incorrect, and was replaced by the empirically tested concept of chemical elements . According to Aristotle,
25620-422: The other. These properties are predicated of an actual substance relative to the work it is able to do; that of heating or chilling and of desiccating or moistening. The four elements exist only with regard to this capacity and relative to some potential work. The celestial element is eternal and unchanging, so only the four terrestrial elements account for "coming to be" and "passing away" – or, in
25803-425: The parts of a natural whole exist for the sake of the whole. As Aristotle himself notes, "for the sake of" locutions are ambiguous: " A is for the sake of B " may mean that A exists or is undertaken in order to bring B about; or it may mean that A is for B’s benefit ( An II.4 415b2—3, 20—1); but both types of finality have, he thinks, a crucial role to play in natural, as well as deliberative, contexts. Thus
25986-442: The possibilities that the universe has more dimensions (which is postulated by theories such as string theory) and that its spacetime may have a multiply connected global topology, in analogy with the cylindrical or toroidal topologies of two-dimensional spaces . General relativity describes how spacetime is curved and bent by mass and energy (gravity). The topology or geometry of the universe includes both local geometry in
26169-458: The presence of other masses. A remark by John Archibald Wheeler that has become proverbial among physicists summarizes the theory: "Spacetime tells matter how to move; matter tells spacetime how to curve", and therefore there is no point in considering one without the other. The Newtonian theory of gravity is a good approximation to the predictions of general relativity when gravitational effects are weak and objects are moving slowly compared to
26352-408: The present dark-energy era, it dominates the mass–energy of the universe because it is uniform across space. Two proposed forms for dark energy are the cosmological constant , a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli , dynamic quantities whose energy density can vary in time and space while still permeating them enough to cause
26535-518: The projectile type, and it is rather reminiscent of the principle of inertia , i.e. Newton's first law of motion ." The eldest Banū Mūsā brother, Ja'far Muhammad ibn Mūsā ibn Shākir (800-873), wrote the Astral Motion and The Force of Attraction . The Persian physicist, Ibn al-Haytham (965-1039) discussed the theory of attraction between bodies. It seems that he was aware of the magnitude of acceleration due to gravity and he discovered that
26718-408: The projectile’s flight (see Phys VIII. 10 266b29—267a11). Similarly, in every case of animal generation, there is always some thing responsible for the continuity of that generation, although it may do so by way of some intervening instrument ( Phys II.3 194b35—195a3). The final cause is that for the sake of which something takes place, its aim or teleological purpose: for a germinating seed, it
26901-473: The qualitative at the expense of the quantitative, neglected mathematics and its proper role in physics (particularly in the analysis of local motion), and relied on such suspect explanatory principles as final causes and "occult" essences. Yet in his Physics Aristotle characterizes physics or the "science of nature" as pertaining to magnitudes ( megethê ), motion (or "process" or "gradual change" – kinêsis ), and time ( chronon ) ( Phys III.4 202b30–1). Indeed,
27084-457: The range of the material cause to include letters (of syllables), fire and the other elements (of physical bodies), parts (of wholes), and even premises (of conclusions: Aristotle re-iterates this claim, in slightly different terms, in An. Post II. 11). The formal cause of a thing is the essential property that makes it the kind of thing it is. In Metaphysics Book Α Aristotle emphasizes that form
27267-465: The ratio of the four elements of which they are composed. For example, earth, the heaviest element, and water, fall toward the center of the cosmos; hence the Earth and for the most part its oceans, will have already come to rest there. At the opposite extreme, the lightest elements, air and especially fire, rise up and away from the center. The elements are not proper substances in Aristotelian theory (or
27450-456: The same from all vantage points and has no center. An explanation for why the expansion of the universe is accelerating remains elusive. It is often attributed to the gravitational influence of "dark energy", an unknown form of energy that is hypothesized to permeate space. On a mass–energy equivalence basis, the density of dark energy (~ 7 × 10 g/cm ) is much less than the density of ordinary matter or dark matter within galaxies. However, in
27633-456: The same time, a second observer who is moving relative to the first will see those events happening at different times. The two observers will disagree on the time T {\displaystyle T} between the events, and they will disagree about the distance D {\displaystyle D} separating the events, but they will agree on the speed of light c {\displaystyle c} , and they will measure
27816-477: The same value for the combination c 2 T 2 − D 2 {\displaystyle c^{2}T^{2}-D^{2}} . The square root of the absolute value of this quantity is called the interval between the two events. The interval expresses how widely separated events are, not just in space or in time, but in the combined setting of spacetime. The special theory of relativity cannot account for gravity . Its successor,
27999-401: The second derivative of the cosmic scale factor a ¨ {\displaystyle {\ddot {a}}} has been positive in the last 5–6 billion years. Modern physics regards events as being organized into spacetime . This idea originated with the special theory of relativity , which predicts that if one observer sees two events happening in different places at
28182-461: The speed at which two identically shaped objects sink or fall is directly proportional to their weights and inversely proportional to the density of the medium through which they move. While describing their terminal velocity , Aristotle must stipulate that there would be no limit at which to compare the speed of atoms falling through a vacuum , (they could move indefinitely fast because there would be no particular place for them to come to rest in
28365-444: The speed of an object's motion is proportional to the force being applied (or, in the case of natural motion, the object's weight) and inversely proportional to the density of the medium, he reasoned that objects moving in a void would move indefinitely fast – and thus any and all objects surrounding the void would immediately fill it. The void, therefore, could never form. The " voids " of modern-day astronomy (such as
28548-414: The speed of light, 13.8 billion light-years (4.2 × 10 ^ pc), the proper distance is larger because the edge of the observable universe and the Earth have since moved further apart. For comparison, the diameter of a typical galaxy is 30,000 light-years (9,198 parsecs ), and the typical distance between two neighboring galaxies is 3 million light-years (919.8 kiloparsecs). As an example,
28731-449: The speed of light. The relation between matter distribution and spacetime curvature is given by the Einstein field equations , which require tensor calculus to express. The universe appears to be a smooth spacetime continuum consisting of three spatial dimensions and one temporal ( time ) dimension. Therefore, an event in the spacetime of the physical universe can be identified by
28914-483: The standard model of cosmology, describing a flat , homogeneous universe presently dominated by dark matter and dark energy . The fine-tuned universe hypothesis is the proposition that the conditions that allow the existence of observable life in the universe can only occur when certain universal fundamental physical constants lie within a very narrow range of values. According to this hypothesis, if any of several fundamental constants were only slightly different,
29097-433: The statistical properties of the universe are the same in all directions as observed from Earth. The universe is bathed in highly isotropic microwave radiation that corresponds to a thermal equilibrium blackbody spectrum of roughly 2.72548 kelvins . The hypothesis that the large-scale universe is homogeneous and isotropic is known as the cosmological principle . A universe that is both homogeneous and isotropic looks
29280-421: The structures they form, from sub-atomic particles to entire galactic filaments . Since the early 20th century, the field of cosmology establishes that space and time emerged together at the Big Bang 13.787 ± 0.020 billion years ago and that the universe has been expanding since then. The portion of the universe that we can see is approximately 93 billion light-years in diameter at present, but
29463-404: The temperature of the universe continued to fall, hadron–anti-hadron pairs were no longer produced. Most of the hadrons and anti-hadrons were then eliminated in particle–antiparticle annihilation reactions, leaving a small residual of hadrons by the time the universe was about one second old. A lepton is an elementary , half-integer spin particle that does not undergo strong interactions but
29646-436: The terms of Aristotle's On Generation and Corruption (Περὶ γενέσεως καὶ φθορᾶς), "generation" and "corruption". The Aristotelian explanation of gravity is that all bodies move toward their natural place. For the elements earth and water, that place is the center of the ( geocentric ) universe; the natural place of water is a concentric shell around the Earth because earth is heavier; it sinks in water. The natural place of air
29829-412: The terrestrial spheres: earth , air , fire and water . He also held that the heavens are made of a special weightless and incorruptible (i.e. unchangeable) fifth element called " aether ". Aether also has the name "quintessence", meaning, literally, "fifth being". Aristotle considered heavy matter such as iron and other metals to consist primarily of the element earth, with a smaller amount of
30012-476: The total matter in the universe. The remaining 4.9% of the mass–energy of the universe is ordinary matter, that is, atoms , ions , electrons and the objects they form. This matter includes stars , which produce nearly all of the light we see from galaxies, as well as interstellar gas in the interstellar and intergalactic media, planets , and all the objects from everyday life that we can bump into, touch or squeeze. The great majority of ordinary matter in
30195-506: The total size of the universe is not known. Some of the earliest cosmological models of the universe were developed by ancient Greek and Indian philosophers and were geocentric , placing Earth at the center. Over the centuries, more precise astronomical observations led Nicolaus Copernicus to develop the heliocentric model with the Sun at the center of the Solar System . In developing
30378-504: The truth. Realism in the sense used by physicists does not equate to realism in metaphysics. The latter is the claim that the world is mind-independent: that even if the results of a measurement do not pre-exist the act of measurement, that does not require that they are the creation of the observer. Furthermore, a mind-independent property does not have to be the value of some physical variable such as position or momentum . A property can be dispositional (or potential), i.e. it can be
30561-423: The universal quality of being human or humanity . The realist school claims that universals are real – they exist and are distinct from the particulars that instantiate them. There are various forms of realism. Two major forms are Platonic realism and Aristotelian realism . Platonic realism is the view that universals are real entities and they exist independent of particulars. Aristotelian realism , on
30744-488: The universe entered a period known as the photon epoch . During this period, the universe was still far too hot for matter to form neutral atoms , so it contained a hot, dense, foggy plasma of negatively charged electrons , neutral neutrinos and positive nuclei. After about 377,000 years, the universe had cooled enough that electrons and nuclei could form the first stable atoms . This is known as recombination for historical reasons; electrons and nuclei were combining for
30927-406: The universe had a beginning and has been expanding since then. According to the Big Bang theory, the energy and matter initially present have become less dense as the universe expanded. After an initial accelerated expansion called the inflationary epoch at around 10 seconds, and the separation of the four known fundamental forces , the universe gradually cooled and continued to expand, allowing
31110-454: The universe has decreased by 1/2 in the past 2 billion years. Today, ordinary matter, which includes atoms, stars, galaxies, and life , accounts for only 4.9% of the contents of the universe. The present overall density of this type of matter is very low, roughly 4.5 × 10 grams per cubic centimeter, corresponding to a density of the order of only one proton for every four cubic meters of volume. The nature of both dark energy and dark matter
31293-406: The universe has expanded monotonically . Perhaps unsurprisingly , our universe has just the right mass–energy density , equivalent to about 5 protons per cubic meter, which has allowed it to expand for the last 13.8 billion years, giving time to form the universe as observed today. There are dynamical forces acting on the particles in the universe which affect the expansion rate. Before 1998, it
31476-482: The universe is unseen, since visible stars and gas inside galaxies and clusters account for less than 10 percent of the ordinary matter contribution to the mass–energy density of the universe. Ordinary matter commonly exists in four states (or phases ): solid , liquid , gas , and plasma . However, advances in experimental techniques have revealed other previously theoretical phases, such as Bose–Einstein condensates and fermionic condensates . Ordinary matter
31659-422: The universe would have been unlikely to be conducive to the establishment and development of matter , astronomical structures, elemental diversity, or life as it is understood. Whether this is true, and whether that question is even logically meaningful to ask, are subjects of much debate. The proposition is discussed among philosophers , scientists , theologians , and proponents of creationism . The universe
31842-467: The universe's density led to concentrations of dark matter gradually forming. Ordinary matter, attracted to these by gravity , formed large gas clouds and eventually, stars and galaxies, where the dark matter was most dense, and voids where it was least dense. After around 100–300 million years, the first stars formed, known as Population III stars. These were probably very massive, luminous, non metallic and short-lived. They were responsible for
32025-450: The vague notion of a common cultural world view , or Weltanschauung . The view that there is a reality independent of any beliefs, perceptions, etc., is called realism . More specifically, philosophers are given to speaking about "realism about " this and that, such as realism about universals or realism about the external world. Generally, where one can identify any class of object, the existence or essential characteristics of which
32208-561: The void). Now however it is understood that at any time prior to achieving terminal velocity in a relatively resistance-free medium like air, two such objects are expected to have nearly identical speeds because both are experiencing a force of gravity proportional to their masses and have thus been accelerating at nearly the same rate. This became especially apparent from the eighteenth century when partial vacuum experiments began to be made, but some two hundred years earlier Galileo had already demonstrated that objects of different weights reach
32391-485: The world, in so far as they rest on the unambiguous and direct contact perception and thought enjoy with their objects. The Aristotelian theory of motion came under criticism and modification during the Middle Ages . Modifications began with John Philoponus in the 6th century, who partly accepted Aristotle's theory that "continuation of motion depends on continued action of a force" but modified it to include his idea that
32574-463: Was a broad field including subjects which would now be called the philosophy of mind , sensory experience , memory , anatomy and biology . It constitutes the foundation of the thought underlying many of his works . Key concepts of Aristotelian physics include the structuring of the cosmos into concentric spheres, with the Earth at the centre and celestial spheres around it. The terrestrial sphere
32757-467: Was aware of. The first is that if the imparted power is less than the resistance, then in reality it will not move the body, but Aristotle's relation says otherwise. Second, what is the source of the increase in imparted power required to increase the velocity of a freely falling body? Third, what is the imparted power that keeps a projectile in motion after it leaves the agent of projection? Aristotle, in his book Physics, Book 8, Chapter 10, 267a 4, proposed
32940-485: Was expected that the expansion rate would be decreasing as time went on due to the influence of gravitational interactions in the universe; and thus there is an additional observable quantity in the universe called the deceleration parameter , which most cosmologists expected to be positive and related to the matter density of the universe. In 1998, the deceleration parameter was measured by two different groups to be negative, approximately −0.55, which technically implies that
33123-536: Was made of four elements , namely earth, air, fire, and water, subject to change and decay. The celestial spheres were made of a fifth element, an unchangeable aether . Objects made of these elements have natural motions: those of earth and water tend to fall; those of air and fire, to rise. The speed of such motion depends on their weights and the density of the medium. Aristotle argued that a vacuum could not exist as speeds would become infinite. Aristotle described four causes or explanations of change as seen on earth:
33306-619: Was needed. An ontological catalogue is an attempt to list the fundamental constituents of reality. The question of whether or not existence is a predicate has been discussed since the Early Modern period, not least in relation to the ontological argument for the existence of God . Existence, that something is, has been contrasted with essence , the question of what something is. Since existence without essence seems blank, it associated with nothingness by philosophers such as Hegel. Nihilism represents an extremely negative view of being,
33489-437: Was never fully endorsed by Niels Bohr , was denounced as mystical and anti-scientific by Albert Einstein . Pauli accepted the term, and described quantum mechanics as lucid mysticism . Universe The universe is all of space and time and their contents. It comprises all of existence , any fundamental interaction , physical process and physical constant , and therefore all forms of matter and energy , and
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