Having determined that a human-being is one, we can look, briefly, at what a human does. The first and most evident characteristics of a human-being are its physical characteristics—humans are material, or physical. It is important to note that, for Aquinas, it cannot be denied that a human person is a material thing. Aquinas, in fact, repudiates Platonic Dualism where the human person is said to be only a soul. Eleonore Stump, in her exposition of Aquinas’s doctrines of forms and bodies, says that in order to really grasp what Aquinas is trying to do, we have “to begin by seeing the depth of Aquinas’s commitment to the view that human beings are material objects and the vehemence with which he rejects what we tend to call ‘Cartesian dualism’.”
Therefore, for Aquinas, a human is a material thing. However, there is more to a human-being than its material composition. This becomes obvious when we notice the difference between a dead human and a living human. The dead, lifeless body of a human has all the same material qualities of a living human, with the exception of those qualities by which we say that a human is alive. It is these qualities that are the most important things to note about living things. It goes without saying that there is a major difference between a lifeless corpse and a living person. If life was caused only by the material elements, then any material thing should be, at least, capable of life, and, at most, living. Therefore, that which causes the living thing to be living, and not inanimate, is something which is over and above the matter of the living thing. This source, or principle, of life was called, by Aristotle, the form. Every living thing is composed of form and matter. The form of the living thing in question is that which determines the type of living thing that the substance is. In other words, we know what a thing is based upon its activity; and its activity flows from its form, otherwise known as its nature. The form, when united with matter, is the cause of the being of the living thing.
As we noted above, a human is characterized, not only by the characteristics of vegetative and sensible life, but also by rationality. As such, not only does he have a form which is the principle of life and activity in him, but his form is also the principle of intellect in him. This rational form is that which gives the human its most distinctive characteristics. The substantial, rational form of human-beings, united with primary matter is what makes a human-being one substance.
We will look, in greater detail, at what matter is, for Aquinas, in a later post. For now, suffice it to say that for Aquinas, matter is formless, it is potency. Aquinas also distinguishes between designated and undesignated matter. Cf. Thomas Aquinas, On Being and Essence, 2nd ed. trans. Armand Maurer (Toronto, ON: Pontifical Institute of Mediaeval Studies, 1968), 36-7.
Eleonore Stump, Aquinas (London: Routledge, 2003), 193.
Though we do not have the space to fully discuss the distinction between accidental and substantial form, we will, at least, explain, briefly, what the distinction is. A substantial form informs prime matter. It is that which a thing is, and by which it is part of a species. Therefore the substantial form of a human person would be that which makes the person a rational animal – a human. Accidental form is found only in an already existing form/matter composite, and does not change what the thing is. A form/matter composite will always have some accidental form, though the accidental forms can be removed and replaced by other accidental forms. Therefore, a human could have many accidental forms, such as, hair or no hair, the way the hair is placed or coloured.
At this point someone might object, what about people with severe brain damage? Some doctors might object, though nowadays not very many, “what about the fetus?” These have no rationality, are they, therefore, no longer human? Aquinas would answer that it is not the actual capacities of the person that makes them human, but the potency for those capacities, by which, though the “matter” of the person may inhibit their full capacity, the form is still human. Therefore, the fetus and the handicapped people are human because of their form, not because of their capacities or functions. Feser notes that “what is essential to a thing remains essential to it even if it is somehow prevented from manifesting itself…And human beings are essentially rational animals even though human beings who are not yet fully formed and those who have been damaged might be prevented from manifesting their rationality.” (Feser, 140.)