The Seventh Argument:

Those who held that the world is eternal, proceeding from an eternal cause, nevertheless maintained that it is contingent in itself: it does not possess existence of itself, but derives its existence solely from its originator. They thus described an existent that has always existed and whose existence is necessary through another as contingent in existence. In this they departed from the position of their predecessors and from the view generally held by mankind: that only what is nonexistent can be contingent, and that nothing can conceivably either exist or not exist unless it was once nonexistent. This was the position of Aristotle and the ancient philosophers, but Avicenna and his followers disagreed with them. Averroes and others criticized them for this, saying:

The contingent can only be conceived as that whose existence and nonexistence are both possible. It may therefore either exist or be nonexistent—that is, remain in nonexistence.

For this reason, they said:

The contingent (1) must have a substrate, just as it is said that a woman may conceive (2), the earth may yield growth, and a child may learn. The substrates in which these possibilities reside are the womb, the earth, and the heart. Thus, in these substrates (3) there may arise the things for which they are receptive: crops, offspring, and knowledge.

But how can something that has always existed and will always exist—whether through itself or through another—be said to be capable of existing and capable of not existing?

If it is said:

“In respect of its essence, it admits either alternative,”

[It is said] (4): If by “its essence” you mean what exists in external reality, then it does not admit both alternatives, for an existence made necessary through another does not admit nonexistence—unless they mean that it admits of being annihilated after having existed. In that case, it would not be perpetually necessary through another, for whenever it admits nonexistence—