Saint Thomas Aquinas, famous philosopher and theologian of the thirteenth century, is known for adhering to a worldview very dependent upon Aristotle’s philosophy. Though the task of reconciling Aristotle’s philosophy with Aquinas’ proved difficult, he was very thorough and well argued in it. One of the biggest problems Aquinas ran into during this attempt to make his views compatible with Aristotle was that of life after death. Aquinas made arguments against Aristotle’s idea that the soul is mortal, among which include the idea of concept abstraction and the necessity of unity between body and soul.
Among Aquinas’ weaker arguments for the immortality of the soul is that of desire. This argument essentially states that because no human desire is in vain, that the desire to live on after death will not either. This argument does not come with great support other than the belief that God does not leave any desire unanswered. A stronger argument comes when Aquinas alludes to concept formation. Aquinas believed that because humans are capable of thinking of ideas apart from material substances, that souls were also able to live apart from the body. For example, we can think of the Pythagorean theory as an abstract concept without seeing it being used to find the length of a hypotenuse. Therefore, the soul can live on without the body because it does not need the body to exist, just like ideas do not need material bodies to be realized.
Aquinas and Aristotle’s views differed greatly on the nature of life after death. Aristotle believed the soul to be the “formal cause” of the human body, that which makes a human namely that. While Aquinas agreed on this account, one thing the two philosophers did not agree on is the permanency of the soul. Aristotle believed the soul was mortal; that it dies with the body. To reconcile this Aristotelian notion of the soul with the Christian doctrine that souls are immortal, Aquinas needed to provide a convincing argument as to why Aristotle was wrong.
Aquinas asserted that the soul could live on after death, and to understand this claim we must first understand how he differentiates between different substances. Aquinas believed there to be “substances” and “subsistents”. The Stanford Encyclopedia of Philosophy explains this very well:
A subsistent is something capable of existing on its own, not in another. But that capacity to exist own its own is not distinctive of a substance. A chair subsists. But on Aquinas’ account, it is not a substance. […] A substance, on the other hand, is something that is both subsistent and complete in a nature—a nature being an intrinsic principle of movement and change in the subject. A detached human hand, while subsistent, is not a substance because it is not complete in a nature. A human hand is defined functionally as part of a human substance. A detached human hand is the remains of a human hand properly speaking, and is only called human analogously. So it is subsistent but not a substance. (McInerny, SEP)
We can conclude from this that a soul is a subsistent. While it can exist on its own, it is not “complete in nature”, as its complete nature is to be the form of the human body. Likewise, Aquinas believed that the human body took on a “subsistent” state after death, and after a time the two would, out of necessity, be joined together again in resurrection of the body into the afterlife. In this light, Aquinas does a fairly good job reconciling Aristotle’s belief that the soul is the form of the body. Aquinas uses this idea to contend that they cannot exist long exclusively, and must be joined together even after death.
While Aquinas’ first two arguments explored above are not extremely convincing, his third argument proves to be fairly persuasive. Aquinas fails to explain the crux of the matter in exactly how the body and soul are resurrected, but for his time in the thirteenth century made great leaps in order to construct a more compatible bridge between his theological beliefs of life after death and Aristotle’s philosophical convictions of the soul dying with the body. While these views do not hold up well with contemporary philosophers who want explanations of casual interaction, Aquinas’ explanation provided rather convincing evidence of the soul living on after death for his time.