Desmond asked:
Avicenna is well known as the author of 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. Which is the premise in this statement?
Answer by Jürgen Lawrenz
The Muslims of Avicenna’s era had Aristotle’s writings to work with; and this whole proof is simply lifted from him. However, it is not conclusive, as the monotheist Avicenna (and the Christian Aquinas in his footsteps) took on board the ‘infinite chain of locomotive causes’ which seems compellingly to end in an atemporal uncaused cause, but ignored that the latter is not ‘locomotive’.
These two words, gently reminding us of the need for an interface, comprise the cardinal hinge… it’s the same issue we encounter in several other problematic ultimate principles, such as the incompatibility between life and non-life, between mind and muscles. But unlike a theologian, an honest philosopher will keep talking until he’s blue in the face, seeking a viable resolution to an irresolvable dilemma. Avicenna, Aquinas & Co. had only to say ‘yes’ to authoritarian dogma and their case (and their life) was safe.
So the premise behind these and all other ontological proofs is, indeed, the concept of an infinite and timeless entity (‘God’) being charged with creating finite and temporal existents. Moreover ex nihilo, simply on the strength of uttering the words ‘Let there be X’. What kind of an entity this ‘God’ might be, is not up for discussion. The more vague, the better. Which is why, in my view, neither Avicenna nor Aquinas are philosophers, though admittedly endowed with philosophical intellect.
Ask a Philosopher 
This is a good question and a good answer (though I disagree with the broad claims made about theologians and the sharp distinction drawn between these and philosophers), and I do not have a horse in this race, but just want to point out that the cosmological proof described is not Avicenna’s proof, or even very reminiscent of Avicenna’s proof, which, if anything, has more to do with set theory than with causality. As I understand it, Avicenna’s proof is along the lines of defining a “contingent” entity as one that has a cause extrinsic to itself, and then stating that a “necessary” (i.e., non-contingent) entity logically must exist, because the set of all contingent entities either (1) is a necessary entity, in which case a necessary entity exists, or (2) is a contingent entity, in which case it has an extrinsic cause, in which case this extrinsic cause is a necessary entity (since it is extrinsic to the set of all contingent entities), in which case, once again, a necessary entity exists.