Descartes believed that mind and body were two different kinds of substances. Consciousness is the essential property of mind substance. By contrast, extension in length, breadth, and depth is the essential property of bodily or material substance. One of the crucial questions for Descartes is how these two substances interact. How can one kind of substance, which is lacking in physical properties, have any influence on another kind of substance that is physical?

By the time Descartes had reached his sixth and final meditation he had solved all of his first meditation doubts. To recap, he had doubted whether there are material objects, whether God was a deceiver, who would mislead him about what his senses revealed. But having offered up two arguments for the existence of God, Descartes is certain that it would be logically inconsistent for this God — who is a perfect being — to be a deceiver. As such, Descartes feels certain that material objects cause our sensations and that God would not mislead him about this.

Cartesian Dualism and Interaction

Descartes believed that persons are combinations of mental and physical substance. This is known as dualism. But he was faced with a problem of explaining how these two substances can interact to form the union that we call a person. “My soul is not in my body as a pilot in a ship,” Descartes says in meditation six. “I am most tightly bound to it.” Descartes wants to say that when a person acts, a kind of causation is at work. For instance, if you decide to walk across the room, there is a mental act, willing, which then causes you to take the walk. But he must explain the details of this cause-effect relationship.

You can see and understand a material substance affecting another material substance. You see a billiard ball contacting another, or a ship pushing aside water as it moves along in the sea. But you see no examples of something material affecting something immaterial. Nor do you see an example of the reverse. Without an example of such an occurrence, it is unclear how you can ever understand it.

No wonder that Princess Elizabeth of Bohemia, who corresponded frequently with Descartes on the matter, wanted his explanation of the issue.

She wrote:

I beg of you to tell me how the human soul can determine the movement of the animal spirits of the body so as to perform voluntary acts — being as it is merely a conscious substance. For the determination of movements seems always to come about from the moving body's being propelled … but you utterly exclude extension from your notion of soul, and contact seems to be incompatible with a thing's being immaterial.

For some philosophers, a satisfactory answer to the princess's question has never been provided.