Mental Causation

History

The existence of mental causation was generally considered uncontroversial by ancient philosophers. For instance, both Plato and Aristotle, although differing in their views about the nature of the mind, held that agents’ mental states need to be invoked in order to give causal explanations of some of their bodily movements.

The modern debate about mental causation can be traced back to René Descartes and the controversy about his theory of the mind. Descartes held that minds and bodies are two radically different kinds of substance: Minds are substances that are thinking and not spatially extended, whereas bodies are substances that are spatially extended and not thinking. (By a substance in general, Descartes understood something that exists and whose existence does not depend on anything else.) In correspondence, Princess Elizabeth of Bohemia complained to Descartes that she found it unintelligible how his theory could allow minds to cause the motion of bodies. She held that bodies could only be moved by things in spatial contact with them, which ruled out minds as causes of bodily movements because they lacked the required spatial attributes. Although he never resolved the dispute with Princess Elizabeth, Descartes later developed a theory that identified the pineal gland as the locus of mind-body interaction. By moving the pineal gland, he claimed, the mind affects the motion of our animal spirits (an air-like kind of matter), which communicate the impulse to our muscles via the nerves.

Gottfried Wilhelm Leibniz criticized Descartes’ position for being at odds with physics. He held that the law of conservation of momentum was violated if minds affected the motion of bodies in the way envisaged by Descartes. Leibniz's own position denied mind-body interaction altogether. According to his view, different substances never interact, but God created them so that their histories unfold independently in perfect, preestablished harmony.

The Argument from the Causal Completeness of the Physical

In the 21st century, virtually no one endorses Leibniz's doctrine of preestablished harmony or Princess Elizabeth's conception of the motion of bodies. Still, most contemporary philosophers share the spirit of their objections to Descartes, which demands that mental causation fit into our picture of the physical world. One element of this picture is the principle of the causal completeness of the physical, which says that every physical effect has a physical cause (this principle is also called “causal closure of the

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