Nothing except God can be eternal. And this statement is far from impossible to uphold: for it has been shown above (Q19, A4) that the will of God is the cause of things.
Therefore things are necessary according as it is necessary for God to will them, since the necessity of the effect depends on the necessity of the cause (Metaph. v, text 6).
Now it was shown above (Q19, A3), that, absolutely speaking, it is not necessary that God should will anything except Himself.
It is not therefore necessary for the world to be always; and hence it cannot be proved by demonstration. Note that Aristotle's reasons (Phys. viii) are not simply, but relatively, demonstrative -- viz. in order to contradict the reasons of some of the ancients who asserted that the world began to exist in some quite impossible manner. This appears in three ways.
Firstly, because, both in Phys. viii and in De Coelo i, text 101, he premises some opinions, as those of Anaxagoras, Empedocles and Plato, and brings forward reasons to refute them.
Secondly, because wherever he speaks of this subject, he quotes the testimony of the ancients, which is not the way of a demonstrator, but of one persuading of what is probable.
Thirdly, because he expressly says (Topic. i, 9), that there are dialectical problems, about which we have nothing to say from reason, as, "whether the world is eternal."