It is difficult to conceive of eternal things like God and our immortal soul, but this article from Culture of Life does a wonderful job showing St. Thomas Aquinas’ reasoning on the immateriality of the soul.
“In my last brief, I summarized the chilling argument from the 2012 Journal of Medical Ethics (JME) defending the indefensible practice of killing infants after birth (so called “after birth abortion”). I described how the authors argue that neither fetuses nor newborns are persons and therefore can be treated in a subpersonal way (killed). And I noted that the May 2013 issue of the JME continues the appalling conversation over the legitimacy of infanticide.
“I proceeded to introduce an argument of Michael Tooley’s against fetal and infant personhood found in the May 2013 issue. Tooley argues that since the concept of an immaterial soul is unsound, any argument against killing fetuses and infants that depends on the existence of such a soul is also unsound.
“In reply, I pointed out that Tooley paints all defenders of the rational soul in the image of René Descartes’ “substance dualism” and then dismisses the concept of the soul as obviously unsound. I said that although Cartesian dualism is indeed unsound, Thomas Aquinas’ account of the nature of the relationship of body and soul is rationally persuasive and not at all threatened by Tooley’s simplistic caricature.
“In this essay I set forth Aquinas’s account of why the rational soul must be immaterial. The argument is complex and Aquinas’ terminology can be difficult to navigate. I therefore attempt to translate his argument into language more consistent with contemporary idiom. Even still, the argument is rough-going. Nevertheless, I think it will repay a careful consideration.
“Aquinas’s argument for the immateriality of the rational soul proposes no mystifications about ghostly essences floating inside mechanical bodies; nor does it argue that any human act is anything other than the act of an embodied person. It argues only that a certain kind of act that we indisputably carry out—concept formation—cannot be sufficiently accounted for by the acts of any material organ. And so it concludes—as Aristotle, who was not a Christian, also concluded—that the rational soul must not be anything material.
“Aquinas’s argument can be summarized in nine steps:
“1. Human knowing includes the knowing not only of particular entities (this furry orange thing)—called sensory knowing, but also the knowing of kinds (cat)—or intellectual knowing. In other words, we not only have the ability to perceive individual things, but also to know them as particular examples of an abstract kind.
“2. The act by which we know a thing’s kind is the act of forming universalized concepts. We first perceive actual things, say, cats or images of cats; over time we abstract from those perceptions the universalized concept of “cat”. We then apply the concept to concrete instances and so come to know the abstract in the individual: “my furry orange cat Jack.”
“3. Now individual things can only be perceived as individual because they are constituted of matter. It is precisely their material existence that makes them perceptible. Abstracted from their sensible qualities, they are not objects of perception.”