A reader writes:

I was reading your writings about mind and brain, and I was wondering about how IQ relates to all of this. Since IQ seems to have a large heritable component to it, and the only thing that can be inherited genetically is physical traits, does IQ and its heritability pose a threat to mind-body dualism?

It seems to me that someone with an IQ of 75 would have a very different mental experience than someone with an IQ of 145, and that they would also make decisions very differently, which, to me at least, would pose a threat to free will as well, since wouldn’t a certain level of intelligence be required to make decisions freely in a meaningful way?

It’s a very thoughtful question and an excellent point. The widely accepted heritability of IQ — between 57 percent and 80 percent in twin studies — is strong evidence for the materiality of the intellect. If intellectual activity is passed from generation to generation by DNA, then the capacity for abstract thought would seem to be material, and not immaterial nor spiritual.

A Traditional Approach

However, if we adopt the traditional approach to the soul taken by Thomas Aquinas (1225–1274), the inheritance of IQ is quite consistent with the immateriality of the intellect and will. For one thing, much of IQ is heritable, but not all. The correspondence is not 100 percent — the child of two geniuses is not necessarily a genius.

Following the ancient Greek philosopher Aristotle, St. Thomas understood the human soul as a rational soul. On that view, our souls have three types of powers — vegetative, sensitive, and rational powers.

Vegetative powers are basic physiological functions — respiration, circulation, reproduction, etc.

Sensitive powers are the powers of sensation (vision, hearing, etc.), perception (the conscious experience of sensations), locomotion, imagination, memory, and emotions.

Rational powers are abstract powers — intellect (i.e., capacity for abstract reason) and will (i.e., free will).

Vegetative and sensitive powers are material powers caused by the brain — breathing, vision, memory, etc., are obviously caused by the matter of the body, brain, nervous system, etc.

Rational powers — abstract reason and free will — are not caused by the body or brain — they interact with matter but are not themselves generated by matter. They are immaterial powers of the human soul.

The Important Point

However, and this is the important point, the intellect and will are dependent on the sensitive and vegetative powers for their normal function. If you can’t breathe, you can’t think abstractly very well. If your heart doesn’t beat, your free will is quite impaired. If you can’t see, you can’t learn abstract ideas from books. If you can’t hear, you can’t learn abstract concepts by listening to someone. If you have a bad memory, you can’t learn abstract concepts very well because you can’t remember the perceptions and images necessary to evoke abstract thought.

Read the rest at Mind Matters News, published by Discovery Institute’s Walter Bradley Center for Natural and Artificial Intelligence.