The problem as inherited from Descartes (1596-1650)
According to Descartes' completed philosophy, the human mind is a thinking substance, a continuously existing being whose essence or nature is thought. Strictly speaking, it has no location in space. It is nothing physical.
The body, on the other hand, is an extended substance. A body is bounded by shape and occupies a place in such a way as to exclude other bodies from that place.
Descartes' view of human nature as composed of the distinct realities of mind and body makes him the defender of (Cartesian) mind-body dualism.
According to Descartes, the mind often controls the motions of the body by affecting the "animal spirits" in the pineal gland, from which the wishes of the mind are relayed to the muscles. Pains and other particular information about the external world is relayed to the mind in the reverse direction, i.e., from the sense organs (themselves bodies) through the animal spirits to the pineal gland (itself a body), from where the corresponding images or feelings are provoked in the mind.
This view is called interactionism. The mind affects body, and the body affects mind. Descartes, then, would be an interactionist.
Why Descartes' Solution is Unsatisfactory
But how can an immaterial thinking thing that has no physical size or location produce motion in a nonthinking extended thing, and how can an extended material thing produce ideas in a thinking thing that has neither size nor location?
A more serious challenge arises from within modern natural philosophy itself. If, as Newton will insist, the physical universe is a self-contained mechanical system, and the total matter and total energy of the universe is constant, then it is not possible that the mind, separate from the physical universe, can intervene in the universe to cause a new motion.
Descartes' interactionist solution often provoked criticisms.
Other Solutions by 17th Century Philosophers
2. Nicholas Malebranche (1638-1715) proposed a solution known as occasionalism. Physical events, i.e., what happens in extended substance or matter, form a complete causal system unaffected by the human mind. Minds, on the other hand, have their own psychological sequences.
On those occasions when it seems that a decision (mental event) causes a bodily motion (physical event), or a physical collision of a hard solid object with our body (physical event) seems to cause pain in our mind (mental event), what is happening is the result of the coordination of the mental with the physical by the creator of both, i.e., God.
3. Thomas Hobbes (1588-1679) is a materialist. He denies the independent existence of finite minds. Mental events are but a class of physical events. He denies the existence of distinct mental substances. For Hobbes, we think with certain parts of our body. Hobbes affirms the existence of God but says that God is "a most pure, invisible spirit corporeal" (quoted in R. S. Peters, "Hobbes," Encyclopedia of Philosophy [MacMillan, 1967], vol. 4, p. 36).
4. Benedict (Baruch) Spinoza (1632-1677) is a monist. A monist holds that ultimately there is just one thing. For Spinoza, mind and extension are two attributes of the one existing thing, i.e., God. (God may have other attributes, i.e., essential features, but thought and extension are the only ones we can know.)
Spinoza gets to his monism by working from a Cartesian assumption that whatever depends upon a substance is a mode or attribute of that substance. Descartes affirms this principle with respect to mind and body (particular thoughts depend on mind and physical shapes depend on body). But Spinoza takes seriously the Cartesian view that mind and body depend ultimately on one substance in particular. Instead of being independent substances, as Descartes ultimately holds, the mental and the physical are attributes of God.
Particular thoughts are modes (temporally limited aspects) in the mind of God, particular bodies and physical events are modes (temporally limited aspects) in God's extension. Human minds are modes of God's thought, human bodies are modes of God's extension. For Spinoza both extension and mind, in God anyway, are perfect and complete; thus they do not interact. (So far this is similar to Malebranche.) But they are not truly separate either, since mind and body are aspects of one and the same thing. Spinoza's take on the mind-body problem is sometimes called dual-aspect theory.
5. Gottfried von Leibniz (1646-1716) developed his unique philosophy of monads. The "world" (everything that is) is composed an indefinitely large number of mental substances --with varying degrees of consciousness, from clear to confused. These are called monads. My "soul" is a relatively conscious monad; what we take to be our body corresponds to a large number of monads with confused perceptions. God is the infinite monad, creator of the world of less powerful monads. Divine pre-established harmony coordinates the perceptions of different monads. God's role in harmonizing the perceptions of distinct monads is similar to Malebranche's idea that God coordinates material with mental events, but for Leibniz, strictly speaking, there are no bodies.
Solutions Proposed by 18th Century British Thinkers
6. The view of George Berkeley (1685-1753) is often known as phenomenal idealism. (The term is meant to distinguish Berkeley's view from Platonic idealism, which affirms the reality of the eternal Forms or Ideas, and Kant's transcendental or critical idealism.) For Berkeley, there are, strictly speaking, no material substances or bodies. What we call matter is nothing but collections of ideas of sense (ideas in our minds not under our control).
According to Berkeley, besides ideas in our minds, there are finite minds (such as our own) and infinite mental substance (God). God is the cause of the harmony we discover in our ideas of sense (what we naively take to be objects in the material world).
7. As revealed in his youthful work, the Treatise on Human Nature, David Hume (1711-1776) is an epistemological skeptic regarding all kinds of substance. We have no knowledge of material substances and what we call bodies is nothing but collections of ideas not under our control ("impressions"). So far Hume follows Berkeley. But we also have no knowledge of mental substances and what I call "me" is not a mental substance, since it is no enduring thing; it is nothing but a changing collection of ideas and feelings which come and go with different frequencies upon the stage of consciousness. Even the word "stage" here does not refer to any definite thing of which we have an idea.