Property Dualism and Non-Physical Properties

Core Idea

Property dualism claims that while the world may be entirely physical at the substance level, mental properties are irreducibly non-physical. This position attempts to preserve the distinctiveness and causal efficacy of consciousness while avoiding interaction problems plaguing substance dualism.

Explainer

From your study of substance dualism you know its central difficulty: if mind and body are two completely different substances, how do they causally interact? How does an immaterial thought move a physical arm? Property dualism accepts the force of this objection and retreats to a more defensible position — there is only one substance (the physical world), but that substance instantiates two irreducibly different kinds of properties. Your brain is a physical object; but when it produces consciousness, it instantiates mental properties that cannot be fully described in physical terms alone.

The classic motivation comes from the knowledge argument: Mary the color scientist knows all physical facts about color perception while confined to a black-and-white room. When she leaves and sees red for the first time, she learns something new — what it is like to see red. If she learned something new, then the fact she learned was not already captured by the physical facts she knew. Therefore, mental properties like "what it's like to see red" are not identical to any physical properties. This is a property that the physical world has, but that resists purely physical description.

Two major variants pull in opposite directions on the question of mental causation. Epiphenomenalism holds that mental properties are caused by physical states but have no causal power of their own — they are byproducts, like the shadow of a moving hand. The shadow doesn't cause anything; the hand does. Your experience of deciding to reach for a glass accompanies the neural events but doesn't cause your arm to move. Most philosophers find this deeply counterintuitive and empirically suspicious — if mental properties are causally inert, why did evolution produce them? Non-reductive physicalism (associated with Putnam, Davidson, and others) takes the alternative: mental properties are genuinely causally relevant, but their causal role cannot be fully described by reducing them to physical descriptions. A belief that it will rain causes you to grab an umbrella, and that causal story is real even if the belief is also realized by particular brain states.

The sharpest internal tension in property dualism is the exclusion problem, developed by Jaegwon Kim. If every physical effect has a sufficient physical cause, and mental properties are not identical to physical properties, then mental properties appear to be causally redundant. The neural firing that causes your arm to move already has a complete physical explanation — there is no causal gap for the mental property to fill. Kim argued this threatens to reduce property dualism to epiphenomenalism, undermining its main advantage over substance dualism. Responses include arguing that mental properties supervene on physical ones in a way that makes them genuinely causally relevant without being reducible, though the exact mechanism remains contested.

The upshot is that property dualism occupies a precarious but philosophically serious middle ground. It avoids Cartesian interaction problems by denying substance dualism, preserves the intuition that consciousness is something over and above physics, but faces the hard internal challenge of explaining how non-physical properties can do real causal work in a physical world.