Proof of the Truthful

Avicenna distinguishes between a thing that needs an external cause in order to exist – a contingent thing – and a thing that is guaranteed to exist by its essence or intrinsic nature – a necessary existent.[12] The argument tries to prove that there is indeed a necessary existent.[12] It does this by first considering whether the opposite could be true: that everything that exists is contingent. Each contingent thing will need something other than itself to bring it into existence, which will in turn need another cause to bring it into existence, and so on.[12] Because this seemed to lead to an infinite regress, cosmological arguments before Avicenna concluded that some necessary cause (such as God) is needed to end the infinite chain.[13] However, Avicenna's argument does not preclude the possibility of an infinite regress.[12][13]

Instead, the argument considers the entire collection (jumla) of contingent things, the sum total of every contingent thing that exists, has existed, or will exist.[12][13] Avicenna argues that this aggregate, too, must obey the rule that applies to a single contingent thing; in other words, it must have something outside itself that causes it to exist.[12] This cause has to be either contingent or necessary. It cannot be contingent, though, because if it were, it would already be included within the aggregate. Thus the only remaining possibility is that an external cause is necessary, and that cause must be a necessary existent.[12]

Avicenna anticipates that one could reject the argument by saying that the collection of contingent things may not be contingent. A whole does not automatically share the features of its parts; for example, in mathematics a set of numbers is not a number.[14] Therefore, the objection goes, the step in the argument that assumes that the collection of contingent things is also contingent, is wrong.[14] However, Avicenna dismisses this counter-argument as a capitulation, and not an objection at all. If the entire collection of contingent things is not contingent, then it must be necessary. This also leads to the conclusion that there is a necessary existent, the very thing Avicenna is trying to prove. Avicenna remarks, "in a certain way, this is the very thing that is sought".[14]

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The limitation of the argument so far is that it only shows the existence of a necessary existent, and that is different from showing the existence of God as worshipped in Islam.[5] An atheist might agree that a necessary existent exists, but it could be the universe itself, or there could be many necessary existents, none of which is God.[5] Avicenna is aware of this limitation, and his works contain numerous arguments to show the necessary existent must have the attributes associated with God identified in Islam.[14]

For example, Avicenna gives a philosophical justification for the Islamic doctrine of tawhid (oneness of God) by showing the uniqueness and simplicity of the necessary existent.[15] He argues that the necessary existent must be unique, using a proof by contradiction, or reductio, showing that a contradiction would follow if one supposes that there were more than one necessary existent. If one postulates two necessary existents, A and B, a simplified version of the argument considers two possibilities: if A is distinct from B as a result of something implied from necessity of existence, then B would share it, too (being a necessary existent itself), and the two are not distinct after all. If, on the other hand, the distinction resulted from something not implied by necessity of existence, then this individuating factor will be a cause for A, and this means that A has a cause and is not a necessary existent after all. Either way, the opposite proposition resulted in contradiction, which to Avicenna proves the correctness of the argument.[16] Avicenna argued that the necessary existent must be simple (not a composite) by a similar reductio strategy. If it were a composite, its internal parts would need a feature that distinguishes each from the others. The distinguishing feature cannot be solely derived from the parts' necessity of existence, because then they would both have the same feature and not be distinct: a contradiction. But it also cannot be accidental, or requiring an outside cause, because this would contradict its necessity of existence.[17]

Avicenna derives other attributes of the necessary existent in multiple texts in order to justify its identification with God.[5] He shows that the necessary existent must also be immaterial,[5] intellective,[18] powerful,[5] generous,[5] of pure good (khayr mahd),[19] willful (irada),[20] "wealthy" or "sufficient" (ghani),[21] and self-subsistent (qayyum),[22] among other qualities. These attributes often correspond to the epithets of God found in the Quran.[21][22] In discussing some of the attributes' derivations, Adamson commented that "a complete consideration of Avicenna's derivation of all the attributes ... would need a book-length study".[23] In general, Avicenna derives the attributes based on two aspects of the necessary existent: (1) its necessity, which can be shown to imply its sheer existence and a range of negations (e.g. not being caused, not being multiple), and (2) its status as a cause of other existents, which can be shown to imply a range of positive relations (e.g. knowing and powerful).[24]

Present-day historian of philosophy Peter Adamson called this argument one of the most influential medieval arguments for God's existence, and Avicenna's biggest contribution to the history of philosophy.[4] Generations of Muslim philosophers and theologians took up the proof and its conception of God as a necessary existent with approval and sometimes with modifications.[4] The phrase wajib al-wujud (necessary existent) became widely used to refer to God, even in the works of Avicenna's staunch critics, a sign of the proof's influence.[2] Outside the Muslim tradition, it is also "enthusiastically"[2] received, repeated, and modified by later philosophers such as Thomas Aquinas (1225–1274) and Duns Scotus (1266–1308) of the Western Christian tradition, as well by Jewish philosophers such as Maimonides (d. 1204).[2][4]

Adamson said that one reason for its popularity is that it matches "an underlying rationale for many people's belief in God",[2] which he contrasted with Anselm's ontological argument, formulated a few years later, which read more like a "clever trick" than a philosophical justification of one's faith.[2] Professor of medieval philosophy Jon McGinnis said that the argument requires only a few premises, namely, the distinction between the necessary and the contingent, that "something exists", and that a set subsists through their members (an assumption McGinnis said to be "almost true by definition").[25]

The Islamic Andalusi philosopher Averroes or Ibn Rushd (1126–1198) criticized the argument on its methodology. Averroes, an avid Aristotelian, argued that God's existence has to be shown on the basis of the natural world, as Aristotle had done. According to Averroes, a proof should be based on physics, and not on metaphysical reflections as in the "Proof of the Truthful".[26] Other Muslim philosophers such as Al-Ghazali (1058–1111) attacked the argument over its implications that seemed incompatible with God as known through the Islamic revelation. For example, according to Avicenna, God can have no features or relations that are contingent, so his causing of the universe must be necessary.[26] Al-Ghazali disputed this as incompatible with the concept of God's untrammelled free will as taught in Al-Ghazali's Asharite theology.[27] He further argued that God's free choice can be shown by the arbitrary nature of the exact size of the universe or the time of its creation.[27]

Peter Adamson offered several more possible lines of criticism. He pointed out that Avicenna adopts a piecemeal approach to prove the necessary existent, and then derives God's traditional attribute from it one at a time. This makes each of the arguments subject to separate assessments. Some might accept the proof for the necessary existent while rejecting the other arguments; such a critic could still reject the existence of God.[15] Another type of criticism might attack the proof of the necessary existent itself. Such a critic might reject Avicenna's conception of contingency, a starting point in the original proof, by saying that the universe could just happen to exist without being necessary or contingent on an external cause.[26]

• Adamson, Peter (2013-07-04). "From the necessary existent to God". In Adamson, Peter (ed.). Interpreting Avicenna: Critical Essays. Cambridge University Press. ISBN 978-0-521-19073-2.
• Adamson, Peter (2016). Philosophy in the Islamic World: A History of Philosophy Without Any Gaps. Oxford University Press. ISBN 978-0-19-957749-1.
• Inati, Shams C. (2014). Ibn Sina's Remarks and Admonitions: Physics and Metaphysics: An Analysis and Annotated Translation. Columbia University Press. ISBN 978-0-231-53742-1.
• McGinnis, Jon (2011). "Avicenna on why God is absolutely necessary". In Wippel, John F. (ed.). The Ultimate Why Question: Why is There Anything at All Rather Than Nothing Whatsoever?. Washington, D.C.: Catholic University of America Press. pp. 65–83. ISBN 978-0-8132-1863-2. Preliminary proof is available at McGinnis' website.
• Mayer, Toby (2001). "Ibn Sina's 'Burhan Al-Siddiqin'". Journal of Islamic Studies. Oxford University Press. 12 (1): 18–39. doi:10.1093/jis/12.1.18.
• Rizvi, Sajjad (2009). "Mulla Sadra". In Zalta, Edward N. (ed.). Stanford Encyclopedia of Philosophy.