One of the challenges of a hylomorphic theory is articulating in what sense the soul is a spiritual substance. This is because the soul is the form of the body, even while every single body that exists has some form.
A tree has a form. In fact, the form of a tree is also a soul. A log also has a form in an analogous sense, although not a soul. A log no longer has an underlying unity; this is why it will decay. But it retains some of the organization impressed upon it by its former substantial existence. It now has a derivative formal existence in terms of the underlying matter, the elemental organization which makes it wood. In fact, it doesn’t turn to mush or prime matter, but is a certain kind of matter, even if it lacks a certain perfection of substantial being (unity, independence, internal principle organization). It now exists as the result of a prior, but no longer extent principle of life, unity, activity, and origination. The elements are substantial in a secondary sense.
The tree’s principle was not matter. Otherwise the principle would need a principle of organization. That principle which was the cause of the tree was a form. Yet even while the form of a tree is not material, Thomas, and D.Q. McInerny, following him, refer to the human soul as a “spiritual substance”.1
What do they mean by this and what are its implications? There are three interrelated conclusions that can be drawn about the human soul which concern its operation, its immortality, and its origin.
Because the soul is an intellectual substance, it is rational, it has an operation which is spiritual persay and does not depend on the body, although it is correlated to what is sensed and begins its activity there. Knowing in the strict sense is not a bodily activity and so this substance has an activity which is distinct from and independent of material.
Because it has such an activity and because this activity is not an accident, but the essential property (the specific difference of the human soul), we can posit that such a soul can exist apart from the body. The intellectual soul is immortal. The remarkable nature of our specific difference (rational animal) is that the difference is itself substantial (a rational being can itself be a substance). This is unlike, say, the flying squirrel, flying cannot itself be a substance. This is why medieval described the soul as a substance and the substantial form of a thing.
Finally, because such a power is not a bodily power, but exceeds the capacity of matter and the because cannot be less then the effect, the source each human soul is not material, but God. It cannot be man alone because while our bodies can produce eggs and other material which can be informed by such a soul, our souls do not divide (they are not material), and so cannot reproduce.
But what about forms like plants and animals and the forms which inhere or uphold elemental matter. It seems that such forms are something like the principle or power of matter. There is no mere matter, mere potency out there, but always also some organizing energy and form. But because plants and animals are what they are by means of a power which organizes their matter and because that form in its totality is related to the organization of that matter, their reproduction can be fully natural. That is, a plant can communicate such a principle or power to its offspring by itself.
This does not mean that there is no such thing as a plant soul or animal soul, but that such a soul is not distinct from the unique living organizing principle which is the cause of such life. But we would fall short of describing the human soul (or the person) if we said the human soul is simply that which organizes a human body.2
This is because a person or intellectual (spiritual substance) is that which is most perfect in the universe and that which is most of all an independent being.3 Our perfection is not without a body, and yet it is not a body by which we are perfected.
1 D. Q. McInerny. Philosophical Psychology, Elmhurst, PA: The Alcuin Press, 1999, 316-318.
2 I am drawing from: Summa I, Q. 90 1& 2 http://newadvent.org/summa/1090.htm; Summa I, Q. 115. 2 http://newadvent.org/summa/1115.htm; and Summa I, Q. 118 http://newadvent.org/summa/1118.htm.
3 ST I, q. 29, a. 3