IN THIS PAPER, I PROVIDE A FORMULATION of Thomas Aquinas's account of the nature of human beings for the purpose of comparing it with other accounts in both the history of philosophy and contemporary analytic philosophy. (1) I discuss how his apparently dualistic understanding of the relationship between soul and body yields the conclusion that a human being exists as a unified substance composed of a rational soul informing, that is, serving as the specific organizing principle of, a physical body. I further address the issue of Aquinas's contention that a human rational soul can exist without being united to a body and show how this ability of a human soul. (2) does not contradict the thesis that a human being exists naturally as embodied. I will also respond to two related questions. First, what accounts for the individuation of human beings as distinct members of the human species? Second, what is the principle of identity by which a human being persists through time and change?
According to Aquinas, a human being is a person. Aquinas adopts the definition of person developed by Boethius: "an individual substance of a rational nature." (3) An example of an individual substance (4) is Bill Clinton. As an individual substance, Bill Clinton can be contrasted with humanity, which is not an individual substance, but is the nature in which many individual substances--including Bill Clinton, William Shakespeare, and myself--share.
Being of a rational nature, that is, having an intellective mind, distinguishes human beings from other material substances:
The form and species of a natural thing are known through their
proper operations. Now the proper operation of a human being,
insofar as he is a human being, is to understand and use reason.
Hence the principle of this operation, namely the intellect, must
be that by which a human being is categorized by species. (5)
In general, a person is a being that exists on its own with a specific nature, shared with other beings of its kind, to be rational. (6) A human being is not simply a person, however. In addition to being rational, a human being is a sensitive, living, and corporeal substance. Human beings have a material nature:
It belongs per se to a human being that there be found in him a
rational soul and a body composed of the four elements. So without
these parts a human being cannot be understood, and they must be
placed in the definition of a human being; so they are parts of the
species and form. (7)
Aquinas further distinguishes human beings--from other types of persons--as rational animals:
Animal indeed is predicated of a human being per se, and similarly
rational of animal. Hence this expression, rational animal, is the
definition of a human being. (8)
Aquinas refers to human beings as essentially animal because, through their material bodies, human beings share certain essential qualities with other members of the animal genus. The primary exemplification of such similarity is the capacity for sense perception. A human body, though, is unique among other kinds of animal bodies in that it is organized to support not only the capacity for sense perception but also the capacity for rational thought. The disposition of a human body is determined by its having a rational soul as its substantial form. As a substantial form, a human rational soul is responsible for (1) the esse (being) of a human being, (2) the actualization of the matter composing a human being, and (3) the unity of existence and activity in a human being. (9)
A human soul and the material body of which it is the substantial form are not two separately existing substances. A substantial form is the actualization of a material body. Aquinas asserts,
Body and soul are not two actually existing substances, but from
these two is made one actually existing substance. …