Aquinas theory of knowledge and God relied on a larger framework concerning human nature. First, we will examine the relationship of the soul to the body; then we will examine a power of the soul, the intellect (that distinctive or defining capacity of human beings); finally, we will see how Aquinas runs into problems in trying to make the Aristotelian view on human nature fit into the Christian doctrine of the resurrection.
Aquinas takes inspiration for his theory about the human body from Aristotle’s “On the Soul”; there, Aristotle claims that the soul is whatever differentiates the living from the non-living, the soul is that animating force present in human and animals alike. Aristotle examines what is distinctive about the human soul and the murky way it survives after death. “The Physics” and “The Metaphysics” are the final two texts by Aristotle which inspire Aquinas.
To this, Aquinas roots himself in the tradition which came after Paramedian thought and the Aristotelian response to matter, form, and substance. A soul is the first principal of life in those things in our world which live. A soul then explains why a living thing is a living thing and not merely a glob of living stuff. A soul, however, to Aquinas is not a physical object; otherwise, one would need to ask what makes the soul alive and from this line of thought follows an infinite regress. The soul is likened to heat. Just as heat is an accidental byproduct of fire so is the soul a byproduct, so to speak, of being alive; the soul is a non-physical actuality of a physical object (human beings in this case).
Human souls, however, are different from animal souls. Human souls exist in their own right, human souls can act on their own. What does the soul act on? It understands. To contextualize: in order for a human body to perceive sensation, it must have sense organs (eyes, nose, etc.), but to make sense of that sensation requires no sense organ. Why is because it is the soul which understands, it is the soul which acts as an organ—a non-physical organ—in its own right. The soul is able to make sense of things which are not in its own nature: an eye is only able to make sense of shapes but the soul is able to make sense of everything. The soul does this on its own, it is always working, so it is a substance. Hence, it can act on its own.
So, it follows that the soul does not cease to exist simply because the body ceases to exist. Why is because the soul is a substance whose very nature is to be alive, to be a form, something which cannot be separated from its form. Here, then, is the difference between animal and human souls: animal souls are generated through their body but the intellectual soul of a human being is produced directly by God. The animal soul doesn’t have a reality outside of the body but the human soul is a substance and continues to exist even when its body ceases to exist.
In Aristotelian philosophy, there are two kinds of powers—active and passive.
Active powers enact change while passive powers undergo change. Since the intellect is a power of the soul, we must ask what kind of power it is, active or passive? The answer is that it is both.
Why: the soul is passive in the sense that it moves from potentiality to actuality. We have minds which are blank slates that gradually fill with thought; however, the soul is also active in that it is what writes thoughts on the mental blank slate. It is this active intellect which supersedes Plato in creating the active form of something and makes it universal. Such a view was attractive since it eliminated those tricky Platonic forms while remaining stable in universalizing concepts.
Aquinas, though, runs into problems. As a Christian, he wants the soul to be separable from the body after death. But, as an Aristotelian, he wants the soul to be intricately intertwined with the body. How can he have it both ways? By understanding the resurrection of the soul as a preamble to faith: the soul does not cease to exist simply because the body ceases to exist. It follows, then, that the soul will eventually be re-united with its body; such is a “mystery of faith” in that it cannot be proved by rational reason.
 Normally, I would explain this out in detail as professor Williams has a not insubstantial section of the lecture devoted to explaining this tradition and the response to it. However, today, I am feeling fatigued; so, I will only mention it in brief and focus instead of Aquinas’s response to the Aristotelian response.