What happens when we die? To grasp Saint Thomas Aquinas's thinking on this question, we will need to be clear on at least one thing. This is that he does not believe that human souls survive as complete human beings after we die. He thinks that human beings are both bodily and non-bodily beings. He therefore concludes that the survival of them as nothing but body, or as nothing but what is not bodily, cannot be the survival of them properly speaking.
To understand Thomas on the survival of the human soul, we must forget about notions like that of people surviving their death as complete incorporeal persons, which is certainly the Cartesian view of survival after death, and which is also, perhaps, the most common view among non-philosophers. [Think of movie portrayals of souls leaving the body at death, dressed in pretty much the same clothes, looking pretty much the same as the deceased, except having some sort of translucent, spectral quality.] Thomas’ view is that Fred's soul can survive the death of Fred. But the soul of Fred when he has died is not itself Fred (i.e. Fred's soul, apart from Fred's body, is not Fred).
Yet why should we even say that Fred's soul can survive the death of Fred? At this point it is important to remember that Thomas thinks of the human soul as 'the form of the body' and as something subsisting. For his argument is that, if that is what the human soul is, then the human soul is not something perishable. He holds that for something (e.g. a cow) to perish is for the thing in question to lose its substantial form, to lose what makes it the kind of thing it is. Perishing, for Thomas, is the loss of form, and form is that in terms of which we analyze perishing. He therefore concludes that it makes no sense to speak of form as such perishing. And if the form in question subsists, he reasons, it continues to exist as something subsistent.
Not being a body capable of perishing (as the biological human organism is), and yet being subsistent, the human soul cannot perish. For Thomas, that by virtue of which I understand and think is not the sort of thing which can die as bodies can die. (Ia, 75, 6) Of course, he is perfectly aware that people die and that their bodies perish. But he does not think that this entails that people are totally extinguished. It only entails the destruction of everything which belongs to them as animals.
However, people, for Thomas, are rational, understanding animals, and they are what they are by virtue of what is not material. This aspect of people must, he concludes, be capable of surviving the destruction of what is material. He does not think we can prove that the soul of Fred must survive Fred's death. In his view, whether or not Fred's soul survives the death of Fred will depend on whether God wills to keep it in being, and Thomas does not think that we are in a position to prove that God must do that. For him, therefore, there is no 'proof of the immortality of the soul'. He holds that Fred's soul could, in principle, cease to exist at any time. But he also thinks that it is not the sort of thing of which it makes sense to say that it can perish as bodies can perish.
Yet, for Thomas, neither is it the sort of thing which can survive as a human animal can survive. So the survival of Fred's soul is not the survival of the human being we call 'Fred'. Or, as Thomas puts it, 'my soul is not I'. People, for him, are very much part of the physical world. Take that world away and what you are left with is not a human person. You are not, for example, left with something able to know by means of sense experience. Nor are you left with something able to undergo the feelings or sensations that go with being bodily. On Thomas’ account, therefore, the human soul can only be said to survive as something purely intellectual, as the locus of thought and will.
One implication of this teaching which Thomas draws is that there is no joy or pain in the life of a surviving soul, where joy and pain involve physiological processes and states. 'A disembodied soul', he states, 'does not feel joy and sadness due to bodily desire, but due to intellectual desire, as with the angels.' (Ia, 77, 8.5)
Given all that, of course, one might naturally ask: 'Can I live after my death?' If my soul is not me, and if only my soul survives my death, then it would seem that I cannot really be said to survive my death. Aquinas, however, accepts this conclusion. He does not regard it as ruling out anything he wants to maintain. In his view, the existence of a human soul apart from what is bodily is unnatural. He also thinks that, if I die and only my soul survives, then I do not survive. For my soul is the soul of the individual person that I am. And I am a particular, perishable, bodily individual. Destroy my body, therefore, and the particular person that I am ceases to exist.
On the other hand, however, not everyone who believes in human life after death wishes to conceive of it as survival of something incorporeal. And such is the case with biblical authors such as the evangelists and St Paul. In their scheme of things, life after death is not a matter of what we might call 'the immortality of the soul'. It is a matter of resurrection.
And this is Thomas’ view as well. My soul is not me, he says. But he also believes that it shall be reunited with my body. And then, he thinks, I shall live again. When my soul is reunited with my body, he argues, I shall again be there as the person I am now. In fact, he adds, the soul naturally belongs with the body.
Thomas does not think that human happiness consists in bodily life. He sees it as ultimately lying in the vision of God, which can be enjoyed without the body. 'There can', he says, 'be no complete and final happiness for us save in the vision of God.' But he also holds that something is lacking with respect to happiness in disembodied souls, and that the lack here lies in the absence of the body.
So long as the soul enjoys God without its partner, its desire, though at rest with what it has, still longs for the body to enter in and share. Desire in a disembodied soul is wholly at rest on the part of the object loved, for it possesses what contents it. Yet not on the part of the subject desiring, for the good is not possessed in every manner that can be wished for. Hence when the body is reassumed happiness will grow, not in depth but in extent... Since it is natural for the soul to be united to the body how is it credible that the perfection of the one should exclude the perfection of the other? Let us declare, then, that happiness complete and entire requires the well-being of the body, both before and during its activity. (Ia2ae, 5.4)
In short, if human beings are to be happy after death as human beings, they will need to be raised from the dead in bodily form. They will need to be what Thomas thinks people are now, i.e. human beings, not incorporeal substances. 'Therefore we believe according to our faith in the future resurrection of the dead.' And this, Thomas thinks, means that between what I am now, and what I am to be hereafter, there must be material continuity. Some philosophers speculating on the possibility of personal survival have settled for less than this. They have, for example, said that I can survive if there is some kind of psychological continuity between me now and me hereafter. Others, believing that for people to exist involves bodies existing, have said that I can live again if there is some body or other for me to 'inhabit' or for me to be identified with. For Thomas, however, personal identity requires bodily continuity. For me to live again, he says, there must be a human body. But not just any old body will do if I am to live again.
Just as the same specific form ought to have the same specific matter, so the same numerical form ought to have the same numerical matter. The soul of an ox cannot be the soul of a horse's body, nor can the soul of this ox be the soul of any other ox. Therefore, since the rational soul that survives remains numerically the same, at the resurrection it must be reunited to numerically the same matter. (Compendium Th., 153)
How? Thomas simply does not explain. He does not claim to know what processes must occur in the resurrection of an individual human being. And he does not think that there is any general scientific reason for holding that people survive their death. 'Since the human body substantially dissolves in death', he says, 'it cannot be restored to numerical identity by the action of nature.' But he is absolutely clear that such numerical identity is needed if I am to live again, and, given his belief in the power of God, he finds no objection in principle to believing that it can be brought about.
Since all things, even the very least, are included under divine providence ... the matter composing this human body of ours, whatever form it may take after our death, evidently does not elude the power or the knowledge of God. Such matter remains numerically the same, in the sense that it exists under quantitative dimensions, by reason of which it can be said to be this particular matter, and is the principle of individuation. If then, this matter remains the same, and if the human body is again fashioned from it by divine power, and if also the rational soul which remains the same in its incorruptibility is united to the same body, the result is that identically the same man is restored to life. (Comp.Th., 154)
On this basis Thomas firmly insists on the reality of Christ's resurrection. 'Whatever properties belong to the nature of a human body', he says, 'were totally present in Christ's risen body.' And this, so he adds, means just what it says. It means, for example, that the risen Christ had flesh, bones, blood, 'and other similar elements [which] pertain to the nature of a human body'.
(From Brian Davies, The Thought of Thomas Aquinas, 215-220.)