If someone should argue,
“If the celestial spheres were eternal, their non-existence would be impossible; yet the proofs show that their non-existence is possible, therefore they are not eternal,”
it is impermissible to reply,
“Rather, only that which is eternal in and of itself cannot cease to exist; something that is eternal through another may still pass into non-existence.”
To contend that the spheres are eternal while at the same time asserting that they can fail to exist is invalid for the reason already stated: the element shared by what is necessary in itself and what is necessary through another necessarily entails eternality. Hence, the common property between an eternal being that exists in itself and an eternal being that exists through another demands that both be everlasting. Whoever claims an entity that is eternal by virtue of another yet says that it can nonetheless not exist is advancing a contradiction, as has been shown elsewhere. For this reason, no rational person has ever maintained that something eternal can both exist and not exist in the familiar sense of possibility. What has been claimed, rather, is that such an entity possesses an essence considered in abstraction—so that one can speak of its existence or non-existence as concepts—while its actual existence is rendered necessary by something outside itself.
The unsoundness of that view has been demonstrated elsewhere, and it has been shown to conflict with the consensus of all rational people—even Aristotle and his followers, who regard as “possible” only that which is originated and thus capable of both existence and non-existence. Those who asserted that the spheres are eternal yet capable of existing or not existing—such as Ibn Sīnā and his adherents—fell into self-contradiction. In numerous passages they themselves affirm that the possible, receptive of both existence and non-existence, must be originated, and that whatever is perpetual cannot be described as possible. As for something eternal whose non-existence is impossible, they do not consider it possible, even when its necessity derives from another.
Only a faction of philosophers, headed by Ibn Sīnā and the like, departed from this principle. They sought to reconcile the teaching of their predecessors with the message brought by the Messengers and with the dictates of reason, yet could do so only by contradicting the Messengers, defying reason, rejecting their predecessors where the latter were right, and following them where they erred. Such a stance constitutes unbelief in the revealed religions, in addition to being internally inconsistent and contrary to the judgment of all rational people.
As for their statement,
“The realization of the ruling in a case that shares the common factor while the other attributes found in the original case are absent does not prove that the common factor alone is the effective cause, for it remains possible that the ruling in that case rests on some other cause,”
we reply: this is a mistake, for whenever the ruling is established together with the common factor in