For St. Thomas Aquinas, any material substance (unlike immaterial, for example, angels) is a metaphysical unity made of matter and form. The substantial form is what configures or organizes the matter into this substance and not any other. For a human being, our substantial form is our soul. Human souls are rational. Therefore, according to Aquinas, what makes me a human and not a cat is my substantial form (my rational soul) that organizes matter into a human being (and not a cat). It follows that every living species has a unique substantial form that makes it belong to that species and not another. Human beings have rational souls (a sort of special name for our substantial form), cats have their substantial form of “cat-nature”.
However, if the rational soul is what organizes matter into a human being, what makes me more than just a representative of the human species? What makes me me, an individual person? This is where I find Aquinas’s response very interesting and, in a way, surprisingly modern. What individuates me as a person is this specific embodiment of the rational soul. In other words, my soul can be properly said to be mine only after it has organized this particular matter (my body). The result is not just a human being but also this particular person (me). It is matter (the body) that individuates each person within the soul-organized human species.
This view underlines the importance of both the body and the soul – both the matter and the substantial form that organizes it into one unity. Although according to Aquinas, the human soul continues a disembodied existence after bodily death, it is only one part of a whole that persists and has to be reunited with matter (bodily resurrection) to be a whole substance again. This is true only of humans among all the material substances (so, no luck for cats) and not true of any immaterial substances that are whole without any matter, like angels (who are both unique species and individuated through their form).
Once such human soul is reunited with matter, i.e. it organizes matter once again after a period of disembodied existence, it will still be me and my soul exactly because it used to be a part of the whole embodied unity when it first came into being and organized matter into a human being whose matter “coloured” this soul into becoming my soul.
One of the implications of the importance that Aquinas attributed to matter in his conception of the individuated soul within a metaphysical unity of a human being is, I think, that this position is more open to an empirical approach to gaining knowledge. The human soul is rational and forms matter into a human but since it is only a part of the whole (i.e. my soul is not me, it is a part of me as a particular whole) the other part – my body and its senses – is also an important component of me. Therefore, it seems that I as an individual human being should pay attention both to the rational, intellectual knowledge and to empirical knowledge coming from my senses.
(In this context, I found the following podcast episode on self-awareness by P. Adamson very instructive)
Image credit: By Carlo Crivelli – http://www.nationalgallery.org.uk/paintings/carlo-crivelli-saint-thomas-aquinas, Public Domain, https://commons.wikimedia.org/w/index.php?curid=528367