thumb|Property dualism: the exemplification of two kinds of property by one kind of substance
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. The opposing position is non-reductive physicalism, which hypothetically allows space for free will.
thumb|Huxley explained mental properties as like the steam on a locomotive.
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:
Jackson continued: