"Aquinas maintains that a human is a single material substance. He understands the soul as the form of the body, which makes a human being the composite of the two. Thus, only living, form-matter composites can truly be called human; dead bodies are “human” only analogously. One actually existing substance comes from body and soul. A human is a single material substance, but still should be understood as having an immaterial soul, which continues after bodily death.
Ultimately, humans are animals; the animal genus is body; body is material substance. When embodied, a human person is an “individual substance in the category rational animal.”[79] The body belongs to the essence of a human being. In his Summa theologiae Aquinas clearly states his position on the nature of the soul; defining it as “the first principle of life.”[80] The soul is not corporeal, or a body; it is the act of a body. Because the intellect is incorporeal, it does not use the bodily organs, as “the operation of anything follows the mode of its being.”[81]
The human soul is perfected in the body, but does not depend on the body, because part of its nature is spiritual. In this way, the soul differs from other forms, which are only found in matter, and thus depend on matter. The soul, as form of the body, does not depend on matter in this way.
The soul is not matter, not even incorporeal or spiritual matter. If it were, it would not be able to understand universals, which are immaterial. A receiver receives things according to the receiver’s own nature, so in order for soul (receiver) to understand (receive) universals, it must have the same nature as universals. Yet, any substance that understands universals may not be a matter-form composite. So, humans have rational souls which are abstract forms independent of the body. But a human being is one existing, single material substance which comes from body and soul: that is what Thomas means when he writes that “something one in nature can be formed from an intellectual substance and a body,” and “a thing one in nature does not result from two permanent entities unless one has the character of substantial form and the other of matter.”[82]
The soul is a "substantial form"; it is a part of a substance, but it is not a substance by itself. Nevertheless, the soul exists separately from the body, and continues, after death, in many of the capacities we think of as human. Substantial form is what makes a thing a member of the species to which it belongs, and substantial form is also the structure or configuration that provides the object with the abilities that make the object what it is. For humans, those abilities are those of the rational animal.
These distinctions can be better understood in the light of Aquinas’ understanding of matter and form, a hylomorphic ("matter/form") theory derived from Aristotle. In any given substance, matter and form are necessarily united, and each is a necessary aspect of that substance. However, they are conceptually separable. Matter represents what is changeable about the substance – what is potentially something else. For example, bronze matter is potentially a statue, or also potentially a cymbal. Matter must be understood as the matter of something. In contrast, form is what determines some particular chunk of matter to be a specific substance and no other. When Aquinas says that the human body is only partly composed of matter, he means the material body is only potentially a human being. The soul is what actualizes that potential into an existing human being. Consequently, the fact that a human body is live human tissue entails that a human soul is wholly present in each part of the human."
