From al-Farabi, Avicenna (Ibn Sina) inherited the Neo-Platonic emanationist scheme of existence. Contrary to the classical Muslim theologians, he rejected creation ex nihilo and argued that cosmos has no beginning but is a natural logical product of the divine One. The super-abundant, pure Good that is the One cannot fail to produce an ordered and good cosmos that does not succeed him in time. The cosmos succeeds God merely in logical order and in existence. Consequently, Avicenna is well known as the author of one an important and influential proof for the existence of God. This proof is a good example of a philosopher’s intellect being deployed for a theological purpose, as was common in medieval philosophy. The argument runs as follows:

There is existence, or rather our phenomenal experience of the world confirms that things exist, and that their existence is non-necessary because we notice that things come into existence and pass out of it. Contingent existence cannot arise unless it is made necessary by a cause. A causal chain in reality must culminate in one un-caused cause because one cannot posit an actual infinite regress of causes (a basic axiom of Aristotelian science). Therefore, the chain of contingent existents must culminate in and find its causal principle in a sole, self-subsistent existent that is Necessary. This, of course, is the same as the God of religion.

An important corollary of this argument is Avicenna’s famous distinction between existence and essence in contingents, between the fact that something exists and what it is. It is a distinction that is arguably latent in Aristotle although the roots of Avicenna’s doctrine are best understood in classical Islamic theology or  Kalam. Avicenna’s theory of essence posits three modalities: essences can exist in the external world associated with qualities and features particular to that reality; they can exist in the mind as concepts associated with qualities in mental existence; and they can exist in themselves devoid of any mode of existence. This final mode of essence is quite distinct from existence. Essences are thus existentially neutral in themselves. Existents in this world exist as something, whether human, animal or inanimate object; they are ‘dressed’ in the form of some essence that is a bundle of properties that describes them as composites. God on the other hand is absolutely simple, and cannot be divided into a bundle of distinct ontological properties that would violate his unity. Contingents, as a mark of their contingency, are conceptual and ontological composites both at the first level of existence and essence and at the second level of properties. Contingent things in this world come to be as mentally distinct composites of existence and essence bestowed by the Necessary.

This proof from contingency is also sometimes termed “radical contingency.” Later arguments raged concerning whether the distinction was mental or real, whether the proof is ontological or cosmological. The clearest problem with Avicenna’s proofs lies in the famous Kantian objection to ontological arguments: is existence meaningful in itself? Further, Cantor’s solution to the problem of infinity may also be seen as a setback to the argument from the impossibility of actual infinites.

source: http://www.iep.utm.edu/avicenna/#H6