The Mind of a Body, The Soul of a Man

According to Descartes, man is essentially a thinking thing. It follows, then, that the soul, the thinking thing, is one substance and the body, which is not the thinking thing, is another separate substance. As two distinct substances which are ultimately independent of each other, the immaterial soul interacts with the material body in order to perform the operations proper to man. But how?

(Que the centuries long debate over the mind/body problem.)

So now we've arrived at a moment where the most popular answer is to deny the existence of an immaterial soul. The mind/body problem is thus solved by denying the 'mind' (or soul) part. We now do not have to wonder how the soul and body interact because there is now only one thing.

There is a sense in which this is close to the truth and a sense in which it is monstrously off the mark. Inasmuch as the materialist wants to conceive of the human being as essentially one thing, he maintains an accurate description of reality. However, inasmuch as he wants to conceive of the human being as essentially material, his description of reality is grossly mistaken.

We can say the same for Descartes, who is right to recognize that man has an immaterial soul but is incorrect to understand this soul as a distinct substance. So, we can maintain that a man constitutes a complete substance, not his soul individually or his body individually. We also maintain that the body, or the material, requires an immaterial soul in order to actually be a human being.

Thus we are left with one final question: Who affirms that man is a unity of matter (body) and form (soul)?


The 5th Way and Intelligent Design

Peter Kreeft — Handbook of Christian Apologetics page 5
5. The Design Argument
This sort of argument is of wide and perennial appeal. Almost everyone admits that reflection on the order and beauty of nature touches something very deep within us. But are the order and beauty of the product of intelligent design and conscious purpose? For theists the answer is yes. Arguments for design are attempts to vindicate this answer; to show why it is the most reasonable one to give. They have been formulated in ways as richly varied as the experience in which they are rooted. The following displays the core or central insight.
1. The universe displays a staggering amount of intelligibility, both within the things we observe and in the way these things relate to others outside themselves. That is to say the way they exist and coexist display an intricately beautiful order and regularity that can fill even the most casual observer with wonder. It is the norm in nature for many different beings to work together to produce the same valuable end—for example, the organs in the body work for our life and health. (See also argument 8.)
2. Either this intelligible order is the product of chance or of intelligent design.
3. Not chance.
4. Therefore the universe is the product of intelligent design.
5. Design comes only from a mind, a designer.

This is not the teleological argument, this is closer to the modern arguments in support of ID. In these arguments, the concept of design is unclear and does not apply with any metaphysical certainty. It fails to demonstrate the existence of the God of classical theism with the same metaphysical force that the 5 ways display. In fact, it displays no metaphysical force at all, since ID claims to be scientific in the modern sense, meaning empirically verifiable. If you cannot see the distinction between these two types of arguments, one largely classical and the other modern, then you simply cannot claim to understand what either Aristotle or Aquinas is actually saying.
I am not being antagonistic for the sake of argument. I am saying that the weaknesses in the modern arguments from design are real and ultimately they will fail because they 1) cannot demonstrate with certainty the existence of the God of classical theism and 2) they accept the modern, meaning since Descartes, conception of nature which denies both final and formal causality, the first of which is central to the teleological argument for God’s existence. There is indeed some honor in attempting to show that the world just looks like it was designed intelligently, but the weight of this argument, I would say, has always been rhetorically powerful while being philosophically lacking.
P.S. Kreeft is probably well aware that Aquinas is not simply a forerunner of William Paley and thus knows that the ancient and medieval arguments for the existence of God are strikingly dissimilar in many key respects, but he also writes popularly for a largely modern audience, and I say this as someone who very much enjoys his books and lectures.