There have been numerous proofs for the existence of God, for Ibn Sina, we have to bear in mind that his view was cosmological in character. This means that he would have begun from the universe and existence as we experience it and continued all the way to a First Cause. Philosophers of his era would also have believed in the impossibility of an infinite regress in anything actual in the world.
My previous post mentioned the essence-existence innovation that Ibn Sina had made and how essences are neutral with regards to existence. Also with regards to modality, we remember that everything which has a necessary essence must exist, anything which is impossible has an essence which precludes existence. That with a contingent essence has no reason to lead us to the belief that it exists or does not exist.
When an essence which is contingent (possible) receives existence, then it must receive this from some external cause to it. According to Ibn Sina, when this happens the contingent object becomes necessary, but only through something else. It's not necessary through itself. Therefore, everything which is contingent is necessary and could not be otherwise, similarly what could not possibly exist is impossible and so lacks existence, something which makes Ibn Sina a determinist. So the contingent is in a state of possible until a cause makes it exist, and it becomes necessary, or it turns out that the cause for its being is absent, in which case it becomes impossible.
The question must then be asked, how do we know that there is anything which is necessary of existence through itself and not through another? Basically, could all existents be contingent? For Ibn Sina, the answer is a clear no. He tells us that if we consider all contigents, ALL contingents in existence. Let us say that we have all of them in a set or aggregate. We can then argue as follows:
- Each of these has a cause because it exists and also because it is possible.
- The whole aggregate is possible because it is made up of things which are themselves possible.
- The entire set needs something to keep it in existence, to sustain it.
- No member in this aggregate could be the cause for the whole set
- So the only way the aggregate can exist is through an external cause. One external cause.
Now one could argue that suppose the One itself is also merely necessary through another, wouldn't we just get One+1 and so on? Well, Ibn Sina says that we cannot have an infinite regress of causes of causes of causes. At some point, something will emerge which will stop this regress. It will not be contingent, it exists so it is not impossible, therefore it is necessary. This is what Ibn Sina calls, "The Necessary Existent" (the rest of us mortals just say Allah or God).
So there you have it, all existence must have a necessary existent in order for us to be here. Since we are here, then that is because there is a necessary existent. Therefore there is a necessary existent. Marvelous isn't it? You can understand now why they didn't call him the greatest medieval philosopher the world has ever seen for nothing.
If you want to read more then you can check out the Metaphysics of the Najat, written by The Man himself. Just a small note on what remains of Ibn Sina's work, there is a lot, however there isn't a person alive who has read all the remaining texts that he wrote, he was a prodigous writer and his works would take a lifetime to go through. One of these works which was most famous is called the Canun (قانون) of Medicine which medical practitioners relied on right up until the mid-seventeenth century.