What Is the Difference Between Substance Dualism and Property Dualism?

Direct Answer

Substance dualism and property dualism are the two main forms of dualism — the family of views holding that mind and matter are fundamentally different. They differ over what is dual: two kinds of substance, or two kinds of property within one substance.

Substance dualism, classically formulated by Descartes in the Meditations on First Philosophy (1641), holds that the mind is a thinking, unextended substance (res cogitans) and the body an extended, non-thinking substance (res extensa). Each is a bearer of properties, and each can in principle exist independently of the other. The mind is not a part of the body, and the body is not a part of the mind; they are two complete things. The immediate problem is interaction: how can an immaterial substance act on a material one, and vice versa?

Property dualism, by contrast, holds that there is only one kind of substance — the physical — but that some physical systems have irreducibly mental properties. A brain, which is entirely physical, can have the property of experiencing pain, and that property cannot be identified with, or reduced to, any purely physical property of the brain. The mind is not a separate thing; consciousness is a non-physical feature of certain physical things. The view is "dualist" because it insists on the irreducibility of mental properties, and "monist" in ontology because it recognizes only physical substances.

The difference is visible in how each view handles the mind-body problem. The substance dualist must explain how two independent substances interact — the notorious interaction problem, which Descartes tried, unsuccessfully, to solve by locating the point of contact in the pineal gland. The property dualist avoids the hardest version of this problem: since the brain is physical and consciousness is a property of it, there is no separate substance to interact with. But the property dualist still faces a version of the problem: if mental properties are not physical, how can they have causal powers in a physical world? Many property dualists accept that mental properties are epiphenomenal — caused by the physical but themselves causally inert — a conclusion that strikes many as unacceptable.

Historical Context

Substance dualism has the longer pedigree. Plato separated the soul from the body; Descartes gave the position its definitive modern form and made it the reference point for all later debate. Its problems — interaction, the conservation of physical energy, the apparent dependence of mind on brain — pushed most philosophers toward monism in the eighteenth and nineteenth centuries, but the dualist intuition never died. Hume and Kant attacked the notion of a mental substance from different directions, and by the twentieth century substance dualism had almost no defenders in professional philosophy.

Property dualism emerged from the twentieth-century struggle to accommodate consciousness within a physicalist framework. The failure of the identity theory — which identified mental states with brain states — to account for qualia led some philosophers to conclude that mental properties are real but non-physical. The view was developed in various forms through the late twentieth century and received its most influential contemporary statement in David Chalmers' The Conscious Mind (1996), which argued that conscious experience is a fundamental feature of the world, on a par with mass and charge. Chalmers' formulation — often called naturalistic property dualism — holds that experience is irreducible to the physical yet lawfully correlated with it.

Philosophical Significance

The distinction matters because it changes what is at stake in the mind-body debate. If substance dualism is true, the mind is a separate entity that could in principle survive the destruction of the body — a position with enormous implications for religion, personal immortality, and the ethics of personal identity. If property dualism is true, the mind is not a separate entity; it is a feature of the physical brain. But the irreducibility of experience still means that a complete physical account of the world is incomplete — which is precisely the point of the hard problem of consciousness.

The distinction also structures the contemporary dialectic. Physicalists typically argue against both forms at once, but their arguments are easier against substance dualism, which posits unexplained entities, than against property dualism, which merely denies the reducibility of one kind of property. Panpsychists and illusionists, meanwhile, each define themselves against property dualism: the panpsychist agrees that experience is fundamental but denies that it is restricted to physical systems; the illusionist denies that there are any phenomenal properties at all. Understanding the difference between the two dualisms is therefore a precondition for understanding almost everything else in the philosophy of mind.

Examples & Thought Experiments

The classic case for substance dualism is Descartes' argument from conceivability: he can clearly conceive of his mind existing without his body, and of his body existing without his mind, so they must be distinct substances. The classic objection is interaction: Princess Elisabeth of Bohemia challenged Descartes to explain how an immaterial mind could move a material body, and no answer has ever satisfied everyone.

The classic case for property dualism is the knowledge argument. Mary, a color scientist who has lived her whole life in a black-and-white room, knows every physical fact about color vision; when she leaves the room and sees red, she learns something new — what it is like to see red. If all the physical facts were insufficient to teach her this, then there are facts about experience that are not physical facts, and mental properties are not physical properties.

The philosophical zombie is the shared thought experiment: a being physically identical to a conscious human but lacking experience. If zombies are conceivable — and many find them so — then experience is a further fact over and above the physical. The zombie supports property dualism directly, and substance dualism indirectly: both agree that consciousness is not entailed by the physical, differing only over whether the extra fact is a substance or a property.

Related Concepts

• Dualism and What Is Dualism?
• The hard problem of consciousness and the explanatory gap
• Qualia and philosophical zombies
• Consciousness and materialism
• Panpsychism and functionalism
• Personal identity

Further Learning

• Stanford Encyclopedia of Philosophy, Dualism
• Stanford Encyclopedia of Philosophy, Consciousness
• Stanford Encyclopedia of Philosophy, Zombies
• David Chalmers, The Conscious Mind (Oxford University Press, 1996)