Cosmological argument

A cosmological argument, in natural theology and natural philosophy (not cosmology), is an argument in which the existence of God is inferred from alleged facts concerning causation, explanation, change, motion, contingency, dependency, or finitude with respect to the universe or some totality of objects.[1][2] It is traditionally known as an argument from universal causation, an argument from first cause, or the causal argument. (about the origin). Whichever term is employed, there are three basic variants of the argument, each with subtle yet important distinctions: the arguments from in causa (causality), in esse (essentiality), and in fieri (becoming).

The basic premises of all of these are the concept of causality. The conclusion of these arguments is first cause (for whichever group of things it is being argued must have a cause or explanation), subsequently deemed to be God. The history of this argument goes back to Aristotle or earlier, was developed in Neoplatonism and early Christianity and later in medieval Islamic theology during the 9th to 12th centuries, and re-introduced to medieval Christian theology in the 13th century by Thomas Aquinas. The cosmological argument is closely related to the principle of sufficient reason as addressed by Gottfried Leibniz and Samuel Clarke, itself a modern exposition of the claim that "nothing comes from nothing" attributed to Parmenides.

Contemporary defenders of cosmological arguments include William Lane Craig,[3] Robert Koons,[4] Alexander Pruss,[5] and William L. Rowe.[6]

History

Plato and Aristotle, depicted here in Raphael's The School of Athens, both developed first cause arguments.

Plato (c. 427–347 BC) and Aristotle (c. 384–322 BC) both posited first cause arguments, though each had certain notable caveats.[7] In The Laws (Book X), Plato posited that all movement in the world and the Cosmos was "imparted motion". This required a "self-originated motion" to set it in motion and to maintain it. In Timaeus, Plato posited a "demiurge" of supreme wisdom and intelligence as the creator of the Cosmos.

Aristotle argued against the idea of a first cause, often confused with the idea of a "prime mover" or "unmoved mover" (πρῶτον κινοῦν ἀκίνητον or primus motor) in his Physics and Metaphysics.[8] Aristotle argued in favor of the idea of several unmoved movers, one powering each celestial sphere, which he believed lived beyond the sphere of the fixed stars, and explained why motion in the universe (which he believed was eternal) had continued for an infinite period of time. Aristotle argued the atomist's assertion of a non-eternal universe would require a first uncaused cause – in his terminology, an efficient first cause – an idea he considered a nonsensical flaw in the reasoning of the atomists.

Like Plato, Aristotle believed in an eternal cosmos with no beginning and no end (which in turn follows Parmenides' famous statement that "nothing comes from nothing"). In what he called "first philosophy" or metaphysics, Aristotle did intend a theological correspondence between the prime mover and deity (presumably Zeus); functionally, however, he provided an explanation for the apparent motion of the "fixed stars" (now understood as the daily rotation of the Earth). According to his theses, immaterial unmoved movers are eternal unchangeable beings that constantly think about thinking, but being immaterial, they are incapable of interacting with the cosmos and have no knowledge of what transpires therein. From an "aspiration or desire",[9] the celestial spheres, imitate that purely intellectual activity as best they can, by uniform circular motion. The unmoved movers inspiring the planetary spheres are no different in kind from the prime mover, they merely suffer a dependency of relation to the prime mover. Correspondingly, the motions of the planets are subordinate to the motion inspired by the prime mover in the sphere of fixed stars. Aristotle's natural theology admitted no creation or capriciousness from the immortal pantheon, but maintained a defense against dangerous charges of impiety.

Plotinus, a third-century Platonist, taught that the One transcendent absolute caused the universe to exist simply as a consequence of its existence (creatio ex deo). His disciple Proclus stated "The One is God".

Centuries later, the Islamic philosopher Avicenna (c. 980–1037) inquired into the question of being, in which he distinguished between essence (Mahiat) and existence (Wujud). He argued that the fact of existence could not be inferred from or accounted for by the essence of existing things, and that form and matter by themselves could not originate and interact with the movement of the Universe or the progressive actualization of existing things. Thus, he reasoned that existence must be due to an agent cause that necessitates, imparts, gives, or adds existence to an essence. To do so, the cause must coexist with its effect and be an existing thing.[10]

Steven Duncan writes that it "was first formulated by a Greek-speaking Syriac Christian neo-Platonist, John Philoponus, who claims to find a contradiction between the Greek pagan insistence on the eternity of the world and the Aristotelian rejection of the existence of any actual infinite". Referring to the argument as the "'Kalam' cosmological argument", Duncan asserts that it "received its fullest articulation at the hands of [medieval] Muslim and Jewish exponents of Kalam ("the use of reason by believers to justify the basic metaphysical presuppositions of the faith").[11]

Thomas Aquinas (c. 1225–1274) adapted and enhanced the argument he found in his reading of Aristotle and Avicenna to form one of the most influential versions of the cosmological argument.[12][13] His conception of First Cause was the idea that the Universe must be caused by something that is itself uncaused, which he claimed is that which we call God:

The second way is from the nature of the efficient cause. In the world of sense we find there is an order of efficient causes. There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible. Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the intermediate cause be several, or only one. Now to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false. Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.[14]

Importantly, Aquinas' Five Ways, given the second question of his Summa Theologica, are not the entirety of Aquinas' demonstration that the Christian God exists. The Five Ways form only the beginning of Aquinas' Treatise on the Divine Nature.

Versions of the argument

Argument from contingency

In the scholastic era, Aquinas formulated the "argument from contingency", following Aristotle in claiming that there must be something to explain why the Universe exists. Since the Universe could, under different circumstances, conceivably not exist (contingency), its existence must have a cause – not merely another contingent thing, but something that exists by necessity (something that must exist in order for anything else to exist).[15] In other words, even if the Universe has always existed, it still owes its existence to an uncaused cause,[16] Aquinas further said: "... and this we understand to be God."[17]

Aquinas's argument from contingency allows for the possibility of a Universe that has no beginning in time. It is a form of argument from universal causation. Aquinas observed that, in nature, there were things with contingent existences. Since it is possible for such things not to exist, there must be some time at which these things did not in fact exist. Thus, according to Aquinas, there must have been a time when nothing existed. If this is so, there would exist nothing that could bring anything into existence. Contingent beings, therefore, are insufficient to account for the existence of contingent beings: there must exist a necessary being whose non-existence is an impossibility, and from which the existence of all contingent beings is derived.

The German philosopher Gottfried Leibniz made a similar argument with his principle of sufficient reason in 1714. "There can be found no fact that is true or existent, or any true proposition," he wrote, "without there being a sufficient reason for its being so and not otherwise, although we cannot know these reasons in most cases." He formulated the cosmological argument succinctly: "Why is there something rather than nothing? The sufficient reason ... is found in a substance which ... is a necessary being bearing the reason for its existence within itself."[18]

Leibniz's argument from contingency is one of the most popular cosmological arguments in philosophy of religion. It attempts to prove the existence of a necessary being and infer that this being is God. Alexander Pruss formulates the argument as follows:

  1. Every contingent fact has an explanation.
  2. There is a contingent fact that includes all other contingent facts.
  3. Therefore, there is an explanation of this fact.
  4. This explanation must involve a necessary being.
  5. This necessary being is God.[19]

Premise 1 is a form of the principle of sufficient reason stating that all contingently true propositions are explained. This is one of the several variants of the PSR which differ in strength, scope, and modal implications. Premise 2 refers to what is known as the Big Conjunctive Contingent Fact (abbreviated BCCF) in philosophy of religion. The BCCF is generally taken to be the totality of all contingent beings or the logical conjunction of all contingent facts.[20] The approach of the argument is that since a contingent fact cannot explain the BCCF, a fact involving a necessary object must be its explanation. Statement 5, which is either seen as a premise or a conclusion, infers that the necessary being which explains the totality of contingent facts is God. In academic literature, several philosophers of religion such as Joshua Rasmussen and T. Ryan Byerly have argued for the inference from (4) to (5).[21]

In esse and in fieri

The difference between the arguments from causation in fieri and in esse is a fairly important one. In fieri is generally translated as "becoming", while in esse is generally translated as "in essence". In fieri, the process of becoming, is similar to building a house. Once it is built, the builder walks away, and it stands on its own accord; compare the watchmaker analogy. (It may require occasional maintenance, but that is beyond the scope of the first cause argument.)

In esse (essence) is more akin to the light from a candle or the liquid in a vessel. George Hayward Joyce, SJ, explained that, "where the light of the candle is dependent on the candle's continued existence, not only does a candle produce light in a room in the first instance, but its continued presence is necessary if the illumination is to continue. If it is removed, the light ceases. Again, a liquid receives its shape from the vessel in which it is contained; but were the pressure of the containing sides withdrawn, it would not retain its form for an instant." This form of the argument is far more difficult to separate from a purely first cause argument than is the example of the house's maintenance above, because here the First Cause is insufficient without the candle's or vessel's continued existence.[22]

Thus, Leibniz's argument is in fieri, while Aquinas' argument is both in fieri and in esse. This distinction is an excellent example of the difference between a deistic view (Leibniz) and a theistic view (Aquinas). As a general trend, the modern slants on the cosmological argument, including the Kalam cosmological argument, tend to lean very strongly towards an in fieri argument.

The philosopher Robert Koons has stated a new variant on the cosmological argument. He says that to deny causation is to deny all empirical ideas – for example, if we know our own hand, we know it because of the chain of causes including light being reflected upon one's eyes, stimulating the retina and sending a message through the optic nerve into your brain. He summarised the purpose of the argument as "that if you don't buy into theistic metaphysics, you're undermining empirical science. The two grew up together historically and are culturally and philosophically inter-dependent ... If you say I just don't buy this causality principle – that's going to be a big big problem for empirical science." This in fieri version of the argument therefore does not intend to prove God, but only to disprove objections involving science, and the idea that contemporary knowledge disproves the cosmological argument.[23]

Kalām cosmological argument

William Lane Craig gives this argument in the following general form:[24]

  1. Whatever begins to exist has a cause.
  2. The Universe began to exist.
  3. Therefore, the Universe has a cause.

Craig explains, by nature of the event (the Universe coming into existence), attributes unique to (the concept of) God must also be attributed to the cause of this event, including but not limited to: enormous power (if not omnipotence), being the creator of the Heavens and the Earth (as God is according to the Christian understanding of God), being eternal and being absolutely self-sufficient. Since these attributes are unique to God, anything with these attributes must be God. Something does have these attributes: the cause; hence, the cause is God, the cause exists; hence, God exists.

Craig defends the second premise, that the Universe had a beginning starting with Al-Ghazali's proof that an actual infinite is impossible. However, If the universe never had a beginning then there would be an actual infinite, an infinite amount of cause and effect events. Hence, the Universe had a beginning.

Metaphysical argument for the existence of God

Duns Scotus, the influential Medieval Christian theologian, created a metaphysical argument for the existence of God. Though it was inspired by Aquinas' argument from motion, he, like other philosophers and theologians, believed that his statement for God's existence could be considered separate to Aquinas'. His explanation for God's existence is long, and can be summarised as follows:[25]

  1. Something can be produced.
  2. It is produced by itself, something or another.
  3. Not by nothing, because nothing causes nothing.
  4. Not by itself, because an effect never causes itself.
  5. Therefore, by another A.
  6. If A is first then we have reached the conclusion.
  7. If A is not first, then we return to 2).
  8. From 3) and 4), we produce another- B. The ascending series is either infinite or finite.
  9. An infinite series is not possible.
  10. Therefore, God exists.

Scotus deals immediately with two objections he can see: first, that there cannot be a first, and second, that the argument falls apart when 1) is questioned. He states that infinite regress is impossible, because it provokes unanswerable questions, like, in modern English, "What is infinity minus infinity?" The second he states can be answered if the question is rephrased using modal logic, meaning that the first statement is instead "It is possible that something can be produced."

Objections and counterarguments

What caused the First Cause?

One objection to the argument is that it leaves open the question of why the First Cause is unique in that it does not require any causes. Proponents argue that the First Cause is exempt from having a cause, while opponents argue that this is special pleading or otherwise untrue.[1] Critics often press that arguing for the First Cause's exemption raises the question of why the First Cause is indeed exempt,[26] whereas defenders maintain that this question has been answered by the various arguments, emphasizing that none of its major forms rests on the premise that everything has a cause.[27]

William Lane Craig, who famously uses the Kalam cosmological argument, argues that the infinite is impossible, whichever perspective the viewer takes, and so there must always have been one unmoved thing to begin the universe. He uses Hilbert's paradox of the Grand Hotel and the question 'What is infinity minus infinity?' to illustrate the idea that the infinite is metaphysically, mathematically, and even conceptually, impossible. Other reasons include the fact that it is impossible to count down from infinity, and that, had the universe existed for an infinite amount of time, every possible event, including the final end of the universe, would already have occurred. He therefore states his argument in three points- firstly, everything that begins to exist has a cause of its existence; secondly, the universe began to exist; so, thirdly, therefore, the universe has a cause of its existence.[28] A response to this argument would be that the cause of the universe's existence (God) would need a cause for its existence, which, in turn, could be responded to as being logically inconsistent with the evidence already presented- even if God did have a cause, there would still necessarily be a cause which began everything, owing to the impossibility of the infinite stated by Craig.

Secondly, it is argued that the premise of causality has been arrived at via a posteriori (inductive) reasoning, which is dependent on experience. David Hume highlighted this problem of induction and argued that causal relations were not true a priori. However, as to whether inductive or deductive reasoning is more valuable still remains a matter of debate, with the general conclusion being that neither is prominent.[29] Opponents of the argument tend to argue that it is unwise to draw conclusions from an extrapolation of causality beyond experience.[1] Andrew Loke replies that, according to the Kalam Cosmological Argument, only things which begin to exist require a cause. On the other hand, something that is without beginning has always existed and therefore does not require a cause. The Cosmological Argument posits that there cannot be an actual infinite regress of causes, therefore there must be an uncaused First Cause that is beginningless and does not require a cause.[30]

Not evidence for a theist God

The basic cosmological argument merely establishes that a First Cause exists, not that it has the attributes of a theistic god, such as omniscience, omnipotence, and omnibenevolence.[31] This is why the argument is often expanded to show that at least some of these attributes are necessarily true, for instance in the modern Kalam argument given above.[1]

Existence of causal loops

A causal loop is a form of predestination paradox arising where traveling backwards in time is deemed a possibility. A sufficiently powerful entity in such a world would have the capacity to travel backwards in time to a point before its own existence, and to then create itself, thereby initiating everything which follows from it.

The usual reason which is given to refute the possibility of a causal loop is it requires that the loop as a whole be its own cause. Richard Hanley argues that causal loops are not logically, physically, or epistemically impossible: "[In timed systems,] the only possibly objectionable feature that all causal loops share is that coincidence is required to explain them."[32] However, Andrew Loke argues that causal loop of the type that is supposed to avoid a First Cause suffers from the problem of vicious circularity and thus it would not work.[33]

Existence of infinite causal chains

David Hume and later Paul Edwards have invoked a similar principle in their criticisms of the cosmological argument. Rowe has called the principle the Hume-Edwards principle:[34]

If the existence of every member of a set is explained, the existence of that set is thereby explained.

Nevertheless, David White argues that the notion of an infinite causal regress providing a proper explanation is fallacious.[35] Furthermore, Demea states that even if the succession of causes is infinite, the whole chain still requires a cause.[36] To explain this, suppose there exists a causal chain of infinite contingent beings. If one asks the question, "Why are there any contingent beings at all?", it does not help to be told that "There are contingent beings because other contingent beings caused them." That answer would just presuppose additional contingent beings. An adequate explanation of why some contingent beings exist would invoke a different sort of being, a necessary being that is not contingent.[37] A response might suppose each individual is contingent but the infinite chain as a whole is not; or the whole infinite causal chain to be its own cause.

Severinsen argues that there is an "infinite" and complex causal structure.[38] White tried to introduce an argument "without appeal to the principle of sufficient reason and without denying the possibility of an infinite causal regress".[39] A number of other arguments have been offered to demonstrate that an actual infinite regress cannot exist, viz. the argument for the impossibility of concrete actual infinities, the argument for the impossibility of traversing an actual infinite, the argument from the lack of capacity to begin to exist, and various arguments from paradoxes.[40]

Big Bang cosmology

Some cosmologists and physicists argue that a challenge to the cosmological argument is the nature of time: "One finds that time just disappears from the Wheeler–DeWitt equation"[41] (Carlo Rovelli). The Big Bang theory states that it is the point in which all dimensions came into existence, the start of both space and time.[42] Then, the question "What was there before the Universe?" makes no sense; the concept of "before" becomes meaningless when considering a situation without time.[42] This has been put forward by J. Richard Gott III, James E. Gunn, David N. Schramm, and Beatrice Tinsley, who said that asking what occurred before the Big Bang is like asking what is north of the North Pole.[42] However, some cosmologists and physicists do attempt to investigate causes for the Big Bang, using such scenarios as the collision of membranes.[43]

Philosopher Edward Feser states that classical philosophers' arguments for the existence of God do not care about the Big Bang or whether the universe had a beginning. The question is not about what got things started or how long they have been going, but rather what keeps them going.[44]

There is also a Big Bang Argument, which is a variation of the Cosmological Argument using the Big Bang Theory to validate the premise that the Universe had a beginning.[45]

See also

References

  1. Reichenbach, Bruce (2012). "Cosmological Argument". The Stanford Encyclopedia of Philosophy (Fall 2006 Edition, Edward N. Zalta (ed.) ed.). Retrieved 4 August 2016.
  2. Oderberg, David S. (September 1, 2007). "The Cosmological Argument". In Meister, Chad; Copan, Paul (eds.). The Routledge Companion to Philosophy of Religion. Routledge. pp. 341–350. ISBN 978-0415380386.
  3. Craig, William Lane; Sinclair, James D. (May 18, 2009). "The Kalam Cosmological Argument". In Craig, William Lane; Moreland, J. P. (eds.). The Blackwell Companion to Natural Theology. Wiley-Blackwell. pp. 101–201. ISBN 978-1405176576.
  4. Koons, Robert (1997). "A New Look at the Cosmological Argument" (PDF). American Philosophical Quarterly. University of Illinois Press. 34 (2): 193–211. Archived from the original (PDF) on 2003-03-14. Retrieved 2015-03-27.
  5. Gale, Richard M.; Pruss, Alexander, eds. (March 2003). The Existence of God. Burlington, VT: Ashgate. ISBN 978-0754620518.
  6. Rowe, William L. (1975). The Cosmological Argument. Princeton University Press. ISBN 978-0691072104.
  7. Craig, WL., The Cosmological Argument from Plato to Leibniz, Wipf and Stock Publishers, 2001, pp. 1–5, 13.
  8. Aristotle, Physics VIII, 4–6; Metaphysics XII, 1–6.
  9. "Cosmological Argument for the Existence of God", in Macmillan Encyclopedia of Philosophy (1967), Vol. 2, p. 233 ff.
  10. "Islam". Encyclopædia Britannica Online. 2007. Retrieved 2007-11-27.
  11. Duncan, S., Analytic philosophy of religion: its history since 1955, Humanities-Ebooks, p.165.
  12. Summa Theologica, St. Thomas Aquinas
  13. Scott David Foutz, An Examination of Thomas Aquinas' Cosmological Arguments as found in the Five Ways Archived 2008-05-09 at the Wayback Machine, Quodlibet Online Journal of Christian Theology and Philosophy
  14. "Summa Theologica I Q2.3". www.newadvent.org.
  15. Summa Theologiae, I: 2, 3
  16. Aquinas was an ardent student of Aristotle's works, a significant number of which had only recently been translated into Latin by William of Moerbeke .
  17. Summa Theologiae, I: 2,3
  18. Monadologie (1714). Nicholas Rescher, trans., 1991. The Monadology: An Edition for Students. Uni. of Pittsburgh Press. Jonathan Bennett's translation. Latta's translation. Archived 2015-11-17 at the Wayback Machine
  19. Quoted from The Blackwell Companion to Natural Theology, The Leibnizian Cosmological Argument, by Alexander R. Pruss, pp.25-6
  20. Oppy, Graham. "On 'a new cosmological argument'". Religious Studies.
  21. Rasmussen, Joshua. "From a Necessary Being to God". International Journal for Philosophy of Religion.
  22. Joyce, George Hayward (1922) Principles of Natural Theology. New York: Longmans Green.
  23. Hainline, Allen (November 22, 2014). "New Proofs for God's Existence". Cross Examined.
  24. Craig, William L. "The Existence of God and the Beginning of the Universe". Truth Journal. Leaderu.com. Retrieved 22 June 2008.
  25. "Authors/Duns Scotus/Ordinatio/Ordinatio I/D2/Q2B - The Logic Museum". www.logicmuseum.com.
  26. Cline, Austin. "Cosmological Argument: Does the Universe Require a First Cause? | Agnosticism/Atheism". About.com. Retrieved June 20, 2008.
  27. Clarke, W. Norris (August 25, 2009). The Creative Retrieval of Saint Thomas Aquinas: Essays in Thomistic Philosophy, New and Old. Fordham Univ Press. ISBN 9780823229307 via Google Books.
  28. Reichenbach, Bruce (September 24, 2019). Zalta, Edward N. (ed.). The Stanford Encyclopedia of Philosophy. Metaphysics Research Lab, Stanford University via Stanford Encyclopedia of Philosophy.
  29. "Deduction & Induction". Socialresearchmethods.net. 2006-10-20. Retrieved 2012-09-02.
  30. Andrew Loke, God and Ultimate Origins (Cham: Springer Nature, 2017), p. 189; Chapter 5.
  31. Cline, Austin (27 July 2015). "Cosmological Argument for the Existence of God". About, Inc. Retrieved 3 August 2016.
  32. Richard Hanley, No End in Sight: Causal Loops in Philosophy, Physics and Fiction, Synthese
  33. Andrew Loke, God and Ultimate Origins (Cham: Springer Nature, 2017), chapter 4.
  34. Alexander R. Pruss, The Hume-Edwards Principle and the Cosmological Argument, International Journal for Philosophy of Religion
  35. White, David E. (1979). "An argument for God's existence". International Journal for Philosophy of Religion. 10 (1–3): 101–115. doi:10.1007/BF00143159.
  36. Calvert, Brian (1983). "Another problem about Part IX of Hume's Dialogues". International Journal for Philosophy of Religion. 14 (2): 65–70. doi:10.1007/BF00131845.
  37. Rota, Michael. "Infinite Causal Chains and Explanation" (DOC). Proceedings of the American Catholic Philosophical Association.
  38. Severinsen, Morten (2001). "Principles Behind Definitions of Diseases – a Criticism of the Principle of Disease Mechanism and the Development of a Pragmatic Alternative". Theoretical Medicine and Bioethics. 22 (4): 319–336. doi:10.1023/A:1011830602137. PMID 11680525. This implies that there is an "infinite" and complex causal structure behind each disease, and that the disease mechanism would have to encompass the whole structure.
  39. White, David E. (1979). "An argument for God's existence". International Journal for Philosophy of Religion. 10 (1–3): 101–115. doi:10.1007/BF00143159. My intention is to show that a cosmological argument for God's existence (not that of a first cause simpliciter) can be constructed without appeal to the principle of sufficient reason and without denying the possibility of an infinite causal regress.
  40. Andrew Loke, God and Ultimate Origins (Cham: Springer Nature, 2017), chapters 2 and 3; Waters, Ben. 2013. "Methuselah's Diary and the Finitude of the Past". Philosophia Christi 15: 463–469; Koons, Robert. 2014. A New Kalam Argument: Revenge of the Grim Reaper. Noûs 48: 256–267.
  41. Folger, Tim. "Time may not exist". Retrieved August 17, 2012.
  42. J. Richard Gott III, James E. Gunn, David N. Schramm, and Beatrice M. Tinsley, "Will the Universe Expand Forever?" Scientific American [March 1976], p. 65
  43. Britt, Robert R. (April 18, 2001). "Brane-Storm | Challenges Part of Big Bang Theory". Space.com. Archived from the original on 11 May 2008. Retrieved June 21, 2008.
  44. Feser, Edward. The Last Superstition, St. Augustine Press 2008, p. 103
  45. An article mentioning the existence of such an argument, but not necessarily endorsing it: http://strangenotions.com/why-we-should-be-cautious-using-the-big-bang-argument
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.