Aristotle gave two principles making a person: a soul and a body. St. Paul, perhaps taking a cue from Plato, gives three: soul, body, and spirit. We can see these accounts as compatible in different ways, but the way that most interests me is by considering that Aristotle’s account of man is taken in the line of what is proper to nature whereas the biblical/ Platonic account requires more than what is proper to nature.
Aristotle’s account of human beings is taken from a book that is part of a larger project of the study of nature, but what Aristotle calls a nature is essentially a principle of motion. Even where the operation of the soul is not a motion (and vital operations are not motions as Aristotle defines the term) he is still chiefly interested in operations so far as they relate to motion and change – sensation explains the animal’s way of getting around the world, and reason is treated so far as it relates to sensation – which is why Aristotle only explains those parts of intellect that relate to the sensible world. But in man there is in addition to this a principle that does not formally relate to motion, but to the immobile, unchangeable, and eternal, that is, the divine. There are some indications that Aristotle recognized this, and he would have seen the need to develop the nature of the principle outside of natural science as such. Taken in this way, we need not regard his account of man in De anima as exhaustive, but as exhaustive of man as a nature, that is, a being that relates intrinsically to the mobile.
Again, form and matter are only of value to one who is questioning about the possibility of motion, that is, a process that is incomplete while on going. For one who is not puzzled by (or at least questioning) motion and becoming, matter and form are only answers to a questions he is not asking. In the measure that one finds immobile realities in nature, matter and form are not the best concepts to turn to. For this reason, there will be problems to address when we use matter and form to explain knowing, life, or even the relation that things have to things that know and are alive. This is not to deny the usefulness of the matter/form binary when explaining these things, but it will more explain them so far as they relate to the mobile, and not so far as they are immobile. So far as we consider life as complete at any moment of the one living, or knowledge as perfect at every moment of the knowing, or anything so far as they are relating to life or knowledge considered in this way, matter and form are not the best binary. We need another reality above and beyond these things – a reality that by its very nature transcends motion and so transcends nature. Spirit suggests itself as an ideal name for such a thing.
Note that, on this account, we have reason to extend the notion of spirit to include even the natural world so far as it is knowable. Spirit in this sense appears to be form so far as it rises above the mere potentiality of matter, and this is the source of the knowability of things, even to sensation.
Spirit can also be considered as synonymous with self. Taken in this way, we could consider matter and form as parts which are used by the self. My soul and my body are principles of my operation, not the self as self. Taken in this way, spirit seems to mean life so far as it is opposed to nature.