Dualism and the mind-body problem

What it is

Dualism is the view that mind and body are fundamentally different kinds of thing.

The strong form, substance dualism, says they are two distinct substances: a physical body extended in space, and a thinking thing that is not. This is the position most religious talk of the soul assumes, and it is what makes survival after death conceptually straightforward — if the mind is not the body, the body's death need not be the mind's.

The weaker form, property dualism, keeps one substance and doubles the properties. There is only physical stuff, but it has two irreducibly different kinds of property: the physical ones physics describes, and experiential ones it does not. This is the version alive in academic philosophy today.

The mind-body problem is the question dualism creates and cannot easily answer: if mind and body are genuinely different in kind, how does either affect the other? Your intention to raise your arm is mental; your arm rising is physical. Something crosses.

Where it came from

Versions are ancient. Plato treats the soul as separable from the body and prior to it. The Indian Samkhya school built a rigorous dualism of purusha (consciousness) and prakriti (nature) that runs on entirely different lines from the European tradition and reaches the interaction problem from another direction — a useful reminder that this is not a distinctively Western puzzle.

The canonical European statement is Descartes' Meditations on First Philosophy (1641), dividing reality into res cogitans, thinking substance, and res extensa, extended substance. Descartes thought the two met in the pineal gland.

The decisive objection arrived almost immediately, in a letter of 1643 from Princess Elisabeth of Bohemia, who asked him plainly how an immaterial thing with no extension and no surface could push a material one — since everything we understand about causing motion involves contact and extension. Descartes' replies are widely regarded as evasive. He never answered her properly, and nobody since has answered her either.

She is worth naming because the standard telling of this story usually reduces her to a footnote. The strongest objection to the most famous theory in modern philosophy of mind was made within two years of publication, by his correspondent, and it still stands.

How it is used

Dualism does enormous quiet work across the territory this site covers. Any claim about a soul leaving the body, surviving it, reincarnating, being contacted after death, or travelling apart from it presupposes some version of it. So does most talk of "energy" as something distinct from the body it animates.

Very often it is doing this work without being named or defended. A great deal of popular spiritual writing is dualist in structure while treating the position as too obvious to require argument — which is precisely when a position is least examined.

In academic philosophy the traffic runs the other way. Substance dualism is a minority position; property dualism, in David Chalmers' hands, is a serious and widely engaged one.

What we can and cannot say

We can say the interaction problem is real and unsolved, and that it is compounded by the causal closure of the physical: as far as physics can tell, every physical event has a sufficient physical cause. If that is right, an immaterial mind has nothing left to do — any gap it might act in appears already filled. A dualist must deny causal closure, or accept epiphenomenalism (the mind exists but changes nothing, which makes it mysterious that we can talk about it), or find a third way. None of these is comfortable.

We can say that substance dualism has lost ground steadily, and that the correlation evidence is the reason. Damage a region of brain and a specific capacity goes — language, face recognition, the ability to form memories, the capacity for moral judgement. Anaesthesia switches experience off chemically. This is exactly what you would expect if the mind depends on the brain, and it takes considerable work to square with a mind that merely uses it.

We cannot say dualism has been refuted. Losing ground is not the same as being disproved, and the conceivability arguments that keep property dualism alive have not been dissolved. The honest position is that the leading alternative, physicalism, has its own unsolved problem at exactly the same spot, which is why the argument is still running after four centuries.

What we can say plainly is this: the fact that dualism is unproven is not evidence for it. Two positions can both be in trouble.

Further reading

• René Descartes, Meditations on First Philosophy (1641) — Meditations II and VI carry the argument.
• The Correspondence between Princess Elisabeth of Bohemia and René Descartes (Lisa Shapiro ed., 2007) — short, sharp, and better than the Meditations on this particular point.
• Howard Robinson, "Dualism", Stanford Encyclopedia of Philosophy — free and even-handed.