Thomas Aquinas

“ But where there are many agents ordered to one another, nothing hinders the power of the higher agent from reaching to the ultimate form; while the powers of the inferior agents extend only to some disposition of matter: thus in the generation of an animal, the seminal power disposes the matter, but the power of the soul gives the form. ”

“ We must therefore say that since the generation of one thing is the corruption of another, it follows of necessity that both in men and in other animals, when a more perfect form supervenes the previous form is corrupted: yet so that the supervening form contains the perfection of the previous form, and something in addition. It is in this way that through many generations and corruptions we arrive at the ultimate substantial form, both in man and other animals. ”

“ Again, the seminal power acts by virtue of the soul of the begetter according as the soul of the begetter is the act of the body, making use of the body in its operation. Now the body has nothing whatever to do in the operation of the intellect. Therefore the power of the intellectual principle, as intellectual, cannot reach the semen. Hence the Philosopher says (De Gener. Animal. ii, 3) : "It follows that the intellect alone comes from without." ”

“ I answer that, Some have maintained that it is accidental to the intellectual soul to be united to the body, asserting that the soul is of the same nature as those spiritual substances which are not united to a body. ”

“ For this reason the Philosopher says that the intellect comes from without. But this will not hold. First, because no substantial form is susceptible of more or less; but addition of greater perfection constitutes another species, just as the addition of unity constitutes another species of number. ”

“ And after the sensitive soul, by the power of the active principle in the semen, has been produced in one of the principal parts of the thing generated, then it is that the sensitive soul of the offspring begins to work towards the perfection of its own body, by nourishment and growth. ”

“ If, however, this union be neither voluntary nor natural on the part of the soul, then it must be the result of some violent cause, and to the soul would have something of a penal and afflicting nature. This is in keeping with the opinion of Origen, who held that souls were embodies in punishment of sin. Since, therefore, all these opinions are unreasonable, we must simply confess that souls were not created before bodies, but are created at the same time as they are infused into them. ”

“ This indeed is apparent to the senses in animals generated from putrefaction. We conclude therefore that the intellectual soul is created by God at the end of human generation, and this soul is at the same time sensitive and nutritive, the pre-existing forms being corrupted. ”

“ In the action of the adulterer, what is of nature is good; in this God concurs. But what there is of inordinate lust is evil; in this God does not concur.
Art. 3 - Whether human souls were created together at the beginning of the world? Objection 1: It would seem that human souls were created together at the beginning of the world. ”