Important to a properly Aristotelian metaphysics is the distinction between actuality and potentiality. Actuality refers to what something is, whereas potentiality refers to what something may become. This distinction is embodied within other sub-distinctions within Aristotelian metaphysics. For example, the distinctions between form and matter, as well as the distinction between substance and accident.
Let us look at an example of the latter distinction. Suppose a cat dies, decomposes beyond recognition, and is summarily devoured by ants. It has undergone a substantial changed. It no longer is what it once was. On the other hand, suppose a child undergoes a growth spurt and grows a few inches. This is not a substantial change. Attributes in the substance have become altered, but the child still is substantially what it was. Something which has the capacity for substantial change, in Aristotelian terminology, is called “matter.”
In order to be capable of change, something must be matter. Matter of a specific form constitutes the matter as a specific substance. A substance is said to possess a certain substantial form, which is the form that makes it the substance that it is. We speak of a changge in matter that nonetheless does not abolish what it is as an “accidental” change. A cat may be declawed, blinded, may age, or undergo any number of a series of changes while still remaining a cat. The only sort of change that would cause it to no longer be a cat is if it were to die.
Each substance is one sort or another of a natural kind. They can undergo accidental changes, which do not alter the type of form something is. Accidental change allows the substance to retain its substantial form. Anything which causes its substantial form to be destroyed, destroys the substance and causes it to become something else.
Prime matter constitutes the substrate of a form. In order for there to be any sort of form, it must be embodied or instantiated in some kind of prime matter. Thus, a form is always concretely embodied, rather than existing beyond time and space, as Aristotle’s mentor, Plato, taught. Prime matter, moreover, is always some kind of form. It does not have to be a specific kind of form, but it is always takes some form of another. A particular body can have accidental or substantial forms.
Aquinas saw the human form as capable of existing without a body. The human soul is the substantial form of the body, but this form can, nonetheless, exist without the body, as it does in the intermediate state after death. There seems to be a tension in Aquinas’ attempt to appropriate Aristotelian hylemorphism for his anthropology, since, for Aristotle, the soul is the substantial form of the human body. Humans are not immortal souls placed into a body that is somehow accidental to them.
Rather, the human soul is the substantial form of the body. Human beings are bodies. Since Aquinas has appropriated this for himself, how can he maintain that human beings can be immortal souls after death? If the human soul is the substantial form of the body, does not the dissolution of the body entail the dissolution of the soul?
For Aquinas, a living being has only one substantial form. It does not have many substantial forms. Suppose that I receive a blood transfusion from someone else. Before the blood is removed from my body, the blood does not have the substantial form of bloodness, or something of this sort. Instead, the form of blood is virtually contained in the form of the human body, such that whatever that liquid was capable of doing by virtue of being what it is as blood, it does by virtue of existing within, and being part of, me as an organic being. It constitutes part of an organic whole which contributes to the human-as-substantial-form, however, rather than being constituted as a substantial form in and of itself.
We do not say that matter has forms. Rather, substances have forms. Matter, in a sense, as Kenny points out (1993), is formless. Rather than possessing form (which, again, belongs only to substances), matter has potentiality. It has the potentiality of taking this or that form. If matter becomes dung, this proves that it had the potentiality of being dung. Indeed, rather than merely possessing potentiality, there is a real sense in which matter is defined precisely in terms of possessing potentiality to become something else. It always takes some form, of course, and as a form, is always some substance, but this substance always has the potential to become something else, and it is by virtue of this potential for change that something is matter.
When matter takes a specific form, it becomes a certain kind of substance because it has taken that form. In the language of Aquinas himself, a form is “that by which…a thing is what it is.” In the language of Kenny (1993), “A substnatial form is that in virtue of which a thing is the kind of thing it is: that, indeed, in virtue fo which it exists at all. An accidental form is that in virtue of which something is F…”.
As we have seen, Aristotle rejects the Platonic understanding of form by which multiple individual substances are constituted as individuals of the same species by virtue of a joint participation in a single form. Instead of three individuals participating in a single Platonic form, they are three individuals, individuated as material entities, and constituted as multiple substantial forms.
As noted before, it is difficult to see how Aquinas could consistently believe that it is by virtue of being material that something is individuated, and still see disembodied humans as individuals. He did indeed insist that such disembodied humans continued to subsist as individuals, but since they were not material, how could they have been individuated? It is not clear that Aquinas is consistent here. Indeed, the human being does not merely have a body, but it is a body, and a living one at that. The dead body is no longer a human body, strictly speaking, at all, but a totally different substance.
Kenny, Anthony (1993). Aquinas on Mind. Routledge Publishing, New York, New York.