For Thomas Aquinas, the soul refers to that which gives life to things. Plants, animals and human beings have souls. Everything that is living has a soul; it is alive precisely because it has a soul. To be alive means to have the ability to reproduce, grow and self-nourish. Plants and animals have these capacities; they also have sensory cognition, appetite and locomotion. Humans also share these capacities, but they also have the power to reason. As Steven Jensen writes, “reason is a special power, unique to human beings by which we are able to understand the immaterial world around us.”1
Aquinas believes that reason does not belong to one organ in the body, like sight is to the eye or taste is to the tongue. He believes that because reason is immaterial it is incorruptible. Sure, our minds help us think, process ideas and know the material world around us, but it also allows us to do more. Reason allows us to know universals, that is, to know “dogs” as oppose to a dog (a particular dog), or to know what “triangle” is, (as opposed to one single triangle). This capacity to know beyond the sensible world does not belong to one organ; it is a capacity that does not exist in the physical body. Because it does not exist in any physical organ (it is not material), it must then be immaterial. Because it is immaterial, it must belong to the soul which is immaterial. Further, it must not diminish along with the body at the moment of death, it must live on with the soul. Jensen writes:
If reason has no organ, then it cannot reside in the whole substance, for a human being is a bodily substance. Rather, reason must reside in some immaterial substrate. Since the soul is responsible for our powers, Thomas identifies that substrate with the human soul. In short, other powers reside in the whole substance-the whole human person-and the whole human being is bodily. In contrast, reason resides in the soul alone.2
Therefore, Thomas Aquinas writes in his answer to Article Six of question 75, Whether the Human Soul is Corruptible?3 that the human soul is self-subsistent and therefore could not be corrupted. The human soul is the form that gives life to the matter (the physical human body). Aquinas writes that “it is impossible for a form to be separated from itself...and therefore it is impossible for a subsistent to cease to exist.” The soul and its capacity to reason, then, will not cease to exist; it is incorruptible. (By Edgar Avendano @latinofilmmaker)
1 Jensen, Steven J., The Human Person: A Beginner’s Thomistic Psychology (Washington D.C.: The Catholic University of America Press, 2018), 176.
2 Jensen, The Human Person, 180.
3 Thomas Aquinas, Summa Theologica, in Thomas S. Hibbs’ Aquinas On Human Nature (Cambridge: Hackett Publishing Company, 1999), 71.