Dante, in Canto XXV, is schooled in the formation of the soul as an individual form having vegetative, sensitive, and rational powers. After refuting the error of Averroes (or one of his errors, at least), the “true” explanation of the soul’s formation is given. This particular and special creation by God of the soul certainly explains the individuality of the human soul, setting it at variance with the “one intellect” theory of Averroes and others that interpreted Aristotle’s psychology in such a way.
St. Thomas gives a clear refutation of the theory in the Summa Contra Gentiles, Book II, Chapter 75:
“For just as it belongs to the human soul by its specific nature to be united to a particular species of body, so this particular soul differs only numerically from that one as the result of having a relationship to a numerically different body. In this way are human souls individuated in relation to bodies, and not as though their individuation were caused by bodies; and so the possible intellect, which is a power of the soul, is individuated likewise….”
“…Hence, it does not follow that the intelligible species are numerically one in this or that knower; otherwise, this and that person’s act of understanding would be numerically one, since operation follows upon the form which is the principle of the species…”
Of course, the above quotes demand to be read in context, including the arguments that these excerpts refute (which St. Thomas, of course, always fairly lays out up front).
Nevertheless, this theory of exactly when the individual soul is infused into the human body is still speculative, and has repercussions. Two such are as follows.
For one, St. Thomas is often said to have denied the doctrine of the Immaculate Conception. This is false on two accounts. In the first place, this was not a dogma yet pronounced, and as such, could not be explicitly denied (or at least “rejected”). Also, St. Thomas himself gives his theory as a speculative, not definitive, teaching. He gives it as reasoned opinion, saying it best fits his understanding.
How this is related has to do with another speculative theory, that in which the body receives the soul (and thus first becomes a human person) at around 40 days after conception. In this way, Mary could have inherited original sin in her body, and at the infusion of her soul, immediately been freed from all stain of original sin.
A second application of this theory of the late infusion of the soul into the body is by pro-abortion advocates who claim Catholic status. We have seen such “theologians” as Nancy Pelosi try to show that St. Thomas would have been pro-first trimester abortion by this, a fanciful idea, for sure.
But it all returns to the Canto at hand. The real purpose of abortion and the mentality that leads to it is our lust. We seek pleasure without consequences. We seek to separate what God has made one. All human sin is, once again, a distortion of the good things the good God has made for us.