Property Dualism

Property Dualism is the idea that while there is only one kind of substance in the world, physical substance, there are two kinds of properties, mental and physical properties, such that mental properties cannot be reduced to or explained by physical properties alone, even though both kinds of properties are generated by the same physical thing, namely brains. More specifically, property dualism maintains that human persons are entirely physical objects, composed wholly by the constituents of fundamental physics and subject only to the laws of physics, but also they have, at the same time and equally inherent, non-physical properties or aspects, namely mental properties or aspects (thoughts, concepts, ideas) that are not reducible to, and not explainable by, the properties of fundamental physics (and its special science derivatives)—even though all of property dualism's properties must come from those constituents of fundamental physics. Simply, human persons would have nonphysical properties but no nonphysical parts.

According to Dean Zimmerman (following Chalmers), property dualism means that, “For at least some mental states, it is not possible to define, in terms of microphysical properties alone, a physical property common to all individuals in that mental state, and only to them.” Property dualism, then, would be the failure of supervenience, which states that “among all the possible individuals in all the possible worlds, there is no pair with all the same microphysical properties but different mental properties” (Zimmerman, 2005).

Core Model and Supervenience

Zimmerman applies property dualism to two famous questions in philosophy of mind: “It seems easy to imagine physically indiscernible zombies (animate human bodies with no consciousness) or people whose spectrum of color experiences is the reverse of one's own. If genuinely possible, these scenarios show that the mental does not supervene upon the physical” (Zimmerman, 2005).

But in a wholly physical world, how could the mental not supervene upon the physical? How could different mental states arise from precisely the same microphysical states (down to the most fundamental physics)? If mental states can so arise, mustn't something be missing, or arbitrary, in the physical world? If mental states cannot so arise, what then of property dualism?

To oversimplify, property dualism is dualistic only in its deep epistemology, not in its deep ontology, which remains entirely materialistic—consciousness remains wholly the product of brain function. Under property dualism, the mind still comes entirely from the brain, without residue. When super-advanced neuroscience accounts for all that can be known about the brain—though obviously it would be fiendishly complex—will there be nothing left over to explain about the mind?

Yet, property dualism has some mental properties as irreducible, a move that perhaps helps blunt attacks on materialist theories of consciousness. But what does this really mean? How irreducible? Irreducible in practice, for sure. But irreducible in principle? What would an absolute complete science, from fundamental physics to neuroscience, not capture?

It is instructive to compare and contrast Property Dualism and Non-Reductive Physicalism. Both views are similar in their aim to explain the nature of consciousness in a world/universe/reality of only physical substances—there is only one kind of substance, the physical—but, in both views, crucially, mental states are not reducible to purely physical processes. That’s the similarity. The two views differ, perhaps significantly, in how they conceive the relationship between the mental and the physical—which impacts not only the deep nature of consciousness, but also the deep nature of all reality.

Comparison with Non-Reductive Physicalism

Under Property Dualism, there are two kinds of properties in the world/universe/reality: physical properties and non-physical (mental) properties. Everything (including brains) is made of physical substance but mental properties (qualia, consciousness) are not reducible to or fully explainable by physical properties. Even if you know all the physical facts about brain activity, you wouldn’t know what it’s like to smell garlic cooking in olive oil unless you experience it—because the property of ‘garlic-smell’ as experienced is not physical. In other words, mental properties are fundamentally distinct from physical properties, though they may arise from physical systems. Consciousness has unique ontological status.

Under Non-Reductive Physicalism, all substances and properties in the world/universe/reality are physical in nature, but mental properties are not reducible to lower-level physical explanations (like neurons or molecules). This means that while the physical gives rise to higher-level properties—mental states—that cannot be explained away by the lower-level properties. The mind emerges from brain processes, and while we can’t reduce beliefs and desires to neuronal impulses, they are still nothing over and above physical processes. In other words, mental properties are dependent on, caused by, and realized by physical states, but they are described at a different explanatory level. Emergence is key.

It would seem easy to blur, or admix, Property Dualism and Non-Reductive Physicalism in that both recognize real mental states and yet only one kind of substance (physical, material substance)—but each makes a very different fundamental claim. To reiterate, Property Dualism relies on consciousness having special ontological status, while Non-Reductive Physicalism relies on the powers of physical emergence.

Critiques and Evaluations

Philosopher Ralph Weir evaluates the common preference in philosophy of mind for varieties of property dualism over other alternatives to physicalism and certainly over substance dualism. He argues that the standard motivations for property dualism “lead directly to nonphysical substances resembling the soul of traditional metaphysics.” Using the conceivability of modal arguments for zombies and ghosts and critiquing Russellian monist forms of property dualism, he concludes that “if you posit nonphysical properties in response to the mind-body problem, then you should be prepared to posit nonphysical substances as well” (Weir, 2023).

Property dualism is the first subcategory under dualism because it is the most materialistic, the least dualistic, of the bunch. While I appreciate its important role in the development of philosophy of mind, I must admit that I've never had it near top-of-list in the marketplace of fundamental theories.

Philosopher Peter van Inwagen muses that “‘property dualism’ is a very odd name to give it.” His argument clarifies the essence of dualism itself. “If there are non-physical substances, then physical and non-physical substances (a cat and an angel, for example) are clean different kinds of thing. Although they are both substances right enough, the division of the category ‘substance’ into the sub-categories ‘physical’ and ‘non-physical’ is an ontologically significant division. We call Descartes and Plato dualists because they think there are substances in both sub-categories. I would suppose that ‘property dualists’ call themselves dualists because they think that the division of properties into physical and non-physical properties is an ontologically significant division of the category ‘property’, a division as significant as the physical/non-physical division of the category ‘substance’. If this is so, I think that the self-chosen description ‘property dualist’ indicates a metaphysical confusion in the way property dualists conceive of properties” (Van Inwagen, 2007b).

According to philosopher-humanist Raymond Tallis, “Property dualism, as embraced by many neurophilosophers, does not therefore seem fundamentally different from materialism. Mind seems to have lost its status as the ontological equal of matter, applying equal traction to the order of things. Matter is everywhere – it seems to be the default mode of being. Mind, by contrast, is localised, existing only in minute stretches of the material world, courtesy of the behaviour of certain portions of matter. As such, it is not matter’s equal. What is more, its late emergence, such that some matter woke up to itself long after the beginning of the universe, seems to lack explanation…. The relative attractiveness of property dualism may be explained by the sense that properties are less divorced from one another than are different substances. However, it raises, and fails to address, the entirely reasonable question: “Properties of what?” And if the properties are deemed close enough to be housed in the same place, this will not do justice to the depth of the difference between matter and mind – between moving ions and the thought that Paris is the capital of France.”

Nonetheless, unlike much-disparaged substance dualism, property dualism remains a respectable position within philosophy of mind (Zimmerman, 2005).