We also see the connection between immortality and morality though Blondel’s analysis of human action. On the one hand, there is an absolute egoism to human action since any good we seek, whether peculiar or common, is always self referential.* Reference to will defines goodness. On the other hand, the self cannot be an absolute center since there are circumstances in which death is preferable to life.
The reality of soul and the eschatological arc of human life that comes with it are therefore attempts to come to terms with an element that shows itself as integral to human action and so to any possible morality. There is no morality where human life makes no sense, and human life makes no sense except where good is both absolutely self-referential and something conditioning the value of the self. It’s hard to see how anything other than soul can explain how one of the goods of life is the conscious renunciation of life.
Seen from an ontology of action, soul is an attempt to explain how the person both really dies and this death can be a good for him. While St. Thomas is often seen as stressing the element in this idea of soul (i.e. the soul is not I, the separated soul is not a person, etc.) it would be better as seeing him trying to keep Aristotelian categories open to the eschatological and moral imperatives of a broader vision that has to preserve both the terror and true annihilation of death and the fact that this death is either an exaltation and degradation of the person who suffers it. The abstract and overly narrow attempts to force us to choose between a Substance Dualism in which the self is the soul in opposition to some corruptible non-self and a Naturalism in which the self is utterly corruptible overlook what is involved in making the action of this self possible in the lived world.
*I trust that everyone is mature enough to know the difference between being self-referential and being selfish, proud, or narcissistic, or even being “self-interested” in a way that is opposed to being “public minded”.