If one were asked to name Avicenna’s greatest contribution to the history of philosophy, one might reasonably choose his proof of God’s existence. The proof shows that there must be a “necessary existent”(wajib al—u/ujud), an entity which subsists through itself and requires no external cause in order to exist. It is, quite simply, an entity which cannot not exist. This conception of God, and the proof that goes with it, ranks among his most influential ideas. It was taken up, usually with approval (if also with modification), by] ewish philosophers like Maimonides, Christian philosophers like Duns Scotus, and generations of Muslim philosophers and theologians. I Our admiration for Avicenna’s achievement should not, however, blind us to the fact that proving the existence of a necessary existent is different from proving the existence of God.

Avicenna was fully aware of this, as is clear from the version of the proof in the Salvation. 2 In this and other versions, he asks us to consider all the contingent entities together as an aggregate. Avicenna is heading for the idea that there must be a cause outside of this aggregate which explains the existence of contingent things: the necessary existent. He thus has to contend with an alternative possibility, namely, that the aggregate of contingent things is somehow self-caused, rather than caused by something external. He dismisses this as impossible, and then adds,“[even] if it is correct, it is in a certain way the very thing that is sought. For anything that is sufficient to necessitate itself is something existing necessarily.” 3 In other words, as soon as his opponent admits that something necessary exists, Avicenna can declare victory: this is what the proof aims to show. But of course, what the opponent would admit here is not the existence of God. Rather, he would say or imply that the aggregate of contingent things—which we may as well call “the universe”—is itself a necessary existent.