Property dualism describes a category of positions in the philosophy of mind which hold that, although the world is composed of just one kind of substance—the physical kind—there exist two distinct kinds of properties: physical properties and mental properties. In other words, it is the view that at least some non-physical, mental properties (such as thoughts, imagination and memories) exist in, or naturally supervene upon, certain physical substances (namely brains). Substance dualism, on the other hand, is the view that there exist in the universe two fundamentally different kinds of substance: physical (matter) and non-physical (mind or consciousness), and subsequently also two kinds of properties which inhere in those respective substances. Both substance and property dualism are opposed to reductive physicalism. Notable proponents of property dualism include David Chalmers, Christof Koch, and Richard Fumerton. It became prominent in the final decades of the twentieth century and is now the leading alternative to physicalism.
Definition Property dualism posits the existence of one material substance with essentially two different kinds of property: physical properties and mental properties. It argues that there are different kinds of properties that pertain to the only type of substance, the material substance: there are physical properties such as having colour or shape and there are mental properties like having certain beliefs or perceptions.
Epiphenomenalism
Epiphenomenalism is a position in the philosophy of mind on the mind–body problem. It holds that one or more mental states and their properties are by-products (or epiphenomena) of the states of a closed physical system, and are not causally reducible to physical states (do not have any influence on physical states). According to this view, mental properties are as such real constituents of the world, but they are causally impotent; while physical causes give rise to mental properties like sensations, volition, ideas, etc., such mental phenomena themselves cause nothing further - they are causal dead ends. The opposing position is non-reductive physicalism, which hypothetically allows space for free will.
The position is credited to English biologist Thomas Huxley (Huxley 1874), who analogised mental properties to the whistle on a steam locomotive. The position found a level of favor amongst some scientific behaviorists over the next few decades, which then dove in response to the cognitive revolution in the 1960s.
Epiphenomenal qualia
In the papers "Epiphenomenal Qualia" and "What Mary Didn't Know", Frank Jackson made the so-called knowledge argument against physicalism. The thought experiment was originally proposed by Jackson as follows:
Mary is a brilliant scientist who is, for whatever reason, forced to investigate the world from a black and white room via a black and white television monitor. She specializes in the neurophysiology of vision and acquires, let us suppose, all the physical information there is to obtain about what goes on when we see ripe tomatoes, or the sky, and use terms like 'red', 'blue', and so on. She discovers, for example, just which wavelength combinations from the sky stimulate the retina, and exactly how this produces via the central nervous system the contraction of the vocal cords and expulsion of air from the lungs that results in the uttering of the sentence 'The sky is blue'. [...] What will happen when Mary is released from her black and white room or is given a color television monitor? Will she learn anything or not? Jackson continued:
It seems just obvious that she will learn something about the world and our visual experience of it. But then it is inescapable that her previous knowledge was incomplete. But she had all the physical information. Ergo, there is more to have than that, and Physicalism is false.
Other proponents
Saul Kripke Saul Kripke has a well-known argument for some kind of property dualism. Using the concept of rigid designators, he states that if dualism is logically possible, then it is the case.
Let 'Descartes' be a name, or rigid designator, of a certain person, and let 'B' be a rigid designator of his body. Then if Descartes were indeed identical to B, the supposed identity, being an identity between two rigid designators, would be necessary.
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