Book Two — Creation, Chapter 83 — Summa Contra Gentiles
[1] Now, since the same things are found both to begin to be and to end, someone might suppose that, because the human soul will not cease to exist, neither will it have begun to exist, but, on the contrary, has always been. And it would seem possible to prove this by the following arguments.
[2] That which will never cease to be has the power to exist forever. But no such thing can ever be truly said not to be; for the extent of a thing’s existential duration is exactly commensurate with its power of existing. But of every thing which had begun to exist, it is at some time true to say that it is not. Therefore, that which will never cease to exist, at no time begins to be.
[3] Moreover, just as the truth of intelligible things is imperishable, so is that truth, of itself, eternal; because it is necessary, and whatever is necessary is eternal, for what is necessary to be cannot possibly not be. Now, the imperishable being of the soul is demonstrated from the imperishability of intelligible truth. Hence, by the same reasoning, the soul’s eternity can be proved from the eternal being of intelligible both.
[4] Also, a thing that lacks several of its principal parts is not perfect. But, clearly, the principal parts of the universe are intellectual substances, in the genus of which human souls belong, as we have shown above. If every day as many human souls begin to exist as men are born, then, obviously, many of the principal parts of the universe are added to it daily, so that it lacks a multiplicity of things. Consequently, the universe is imperfect. But this is impossible.
[5] Then, too, some draw their arguments from the authority of Sacred Scripture. For in Genesis (2:2) it is said that “on the seventh day God ended His work which He had made: and He rested from all His work which He had done.” But, if God made new souls every day, this would not be true. Therefore, no new human souls ever begin to exist, but they have existed from the beginning of the world.
[6] Hence, for these and similar reasons, proponents of the doctrine of the world’s eternity have said that, just as the human soul is incorruptible, so has it existed from all eternity. That is why the upholders of the theory of the immortality of human souls in their multiple existence—I refer to the Platonists—asserted that they have existed from eternity, and are united to bodies at one time and separated from them at another, these vicissitudes following a fixed cyclical pattern throughout set periods of years. Advocates of the theory that human souls are immortal in respect of some single reality, pertaining to all men, which remains after death, declared, however, that this one entity has endured from all eternity; whether it be the agent intellect alone, as Alexander held, or, together with this, the possible intellect, as Averroes maintained. Aristotle, also, seems to be making the same point when, speaking of the intellect, he says that it is not only incorruptible, but also everlasting.
[7] On the other hand, some who profess the Catholic faith, yet are imbued with the teachings of the Platonists, have taken a middle position. For, since the Catholic faith teaches that nothing is eternal except God, these persons maintain, not that human souls are eternal, but that they were created with, or rather before, the visible world, yet are fettered to bodies anew. Among these Christians, Origen was the first exponent of this theory, and a number of his disciples followed suit. The theory, indeed, survives to this day among heretics, the Manicheans, for example, siding with Plato in proclaiming the eternity and transmutation of souls.
[8] Now, all these opinions can be easily shown to have no foundation in truth. For it has already been proved that there does not exist only one possible agent intellect for all men. Hence, it remains for us to proceed against those theories which, while envisaging the existence of many human souls, maintain that they existed before bodies, either from eternity, or from the foundation of the world. The incongruity of such a notion is exposed by the following arguments.
[9] For, it has already been established that the soul is united to the body as its form and act. Now, although act is prior in its nature to potentiality, nevertheless in one and the same thing it is temporally posterior to it; for a thing is moved from potentiality to act. Thus, seed, which is potentially living, preceded the soul, which is the act of life.
[10] Moreover, it is natural to every form to be united to its proper matter; otherwise, that which is made of form and matter would be something preternatural. But that which befits a thing naturally is attributed to it before that which befits it preternaturally, because the latter is in it by accident, the former, through itself. Now, that which is by accident is always posterior to that which is through itself. It is, therefore, becoming to the soul to be united to the body before being separated from it. The soul, then, was not created before the body to which it is united.