In the preface to the second edition of his German Metaphysics (1751) Christian Wolff distinguishes three responses to the mind-body problem. Materialism says that everything is ultimately material. Idealism says that everything is ultimately mental. Substance dualism says that there are both material and mental things and that these are different substances. Through Baumgarten and Kant, Wolff’s trichotomy shaped the way later theorists approach the mind-body problem and the Wolffian positions retain a kind of classical status.

When Wolff was writing, Descartes’ substance dualism and Leibniz’s idealism were the leading positions. Materialism was relatively unpopular. This situation has changed remarkably. The present-day incarnation of materialism, physicalism, overtook substance dualism and idealism in the middle of the twentieth century to become the leading response to the mind-body problem. At the same time, substance dualism and idealism have become decidedly unpopular. Even present-day opponents of physicalism tend to reject the classical anti-physicalist positions in favour of standard or Russellian monist forms of property dualism.

For this reason, an important question for present-day philosophy is whether it is really feasible to respond to the mind-body problem by positing nonphysical properties without nonphysical substances. This thesis argues that when the surrounding issues are clarified, both standard and Russellian monist forms of property dualism turn out to be much less plausible than we usually suppose. I conclude that if you posit nonphysical properties in response to the mind-body problem then you should be prepared to posit nonphysical substances as well.

If it is true that the viable anti-physicalist positions require nonphysical substances, the effect on the philosophy of mind ought to be considerable. This would mean a return to the view that something resembling Wolff’s trichotomy exhausts the main responses to the mind-body problem and hence to the status quo of the eighteenth century. Furthermore, if anti-physicalist positions continue to grow in popularity, it means that we must once again take mental substances seriously, however archaic this may seem.