According to Cartesian dualism, the mind is an immaterial substance that do not have spatial extension whereas the body is made up of spatially extended material substance. This dichotomy has created a difficult problem: how can the mind drive body's movement? In a 1643 letter to Descartes, Princess Elizabeth of Bohemia wrote:

"So I ask you please to tell me how the human soul can determine the movement of the animal spirits in the body so as to perform voluntary acts—being as it is merely a conscious substance. For the determination of movement seems always to come about from the moving body's being propelled—to depend on the kind of impulse it gets from what sets it in motion, or again, on the nature and shape of this latter thing's surface. Now the first two conditions involve contact, and the third involves that the impelling thing has extension; but you utterly exclude extension from your notion of soul, and contact seems to me incompatible with a thing's being immaterial..."

Here the word "soul" means "mind". The two words were often used interchangeably at that time.

The Role of Pineal Gland

  

Figure 3. Descartes believed that sensory stimulation might converge at the pineal gland to drive bodily movement.  

Descartes did not directly address the issue raised by Princess Elizabeth. Instead, he proposed that the pineal gland should play a central role in the mind-body interaction. This view was based on the following argument (Stanford Encyclopedia of Philosophy):

"My view is that this gland is the principal seat of the soul, and the place in which all our thoughts are formed. The reason I believe this is that I cannot find any part of the brain, except this, which is not double. Since we see only one thing with two eyes, and hear only one voice with two ears, and in short have never more than one thought at a time, it must necessarily be the case that the impressions which enter by the two eyes or by the two ears, and so on, unite with each other in some part of the body before being considered by the soul. Now it is impossible to find any such place in the whole head except this gland; moreover it is situated in the most suitable possible place for this purpose, in the middle of all the concavities; and it is supported and surrounded by the little branches of the carotid arteries which bring the spirits into the brain."

Descartes postulated that the movement of pineal gland might change the tension of muscles, thereby resulting in bodily movement. The pineal gland could be moved either by sensory stimulation or the force of soul (mind).

The Role of Striatum

Although modern neuroscience did not find any motor system connected to the pineal gland, Descartes' basic idea could be right: there could exist in the brain a specific area responsible for the mind-body interaction. Compelling evidence indicates that such area is the striatum which has been demonstrated to play a central role in action selection. Details are described in Chapter 11. Before that, we should know first what the mind is, and where the feeling of pain and pleasure comes from. Then we could explain the animal behavior, "seek pleasure, avoid pain", on the basis of well-established neural circuits connected to the striatum. Not only can the issue raised by Princess Elizabeth be resolved, the Geon Hypothesis further explains how the pleasant feeling drives the action of "seek" and how the painful feeling propels the movement of "avoid".

According to the Geon Hypothesis, the mind comprises a huge number of gravitational and electromagnetic waves. The next chapter will briefly introduce these waves. Chapter 5 will discuss gravitational waves in more detail.

Author: Frank Lee
Date: 2019-11-5