Piero Scaruffi(Copyright © 2013 Piero Scaruffi | Legal restrictions )
These are excerpts and elaborations from my book "The Nature of Consciousness"
There exist two main brands of dualism: "substance" dualism (the mind is a different substance altogether from the brain), and "property" dualism. According to the latter, the mind is the same substance as the brain, but comes from a class of properties that are exclusive of the brain. The main version of property dualism is "supervenience" theory.
"Supervenience" is used to express the fact that a domain is fully determined by another domain. For example, biological properties "supervene" (or "are supervenient") on physical properties, because the biological properties of a system are determined by its physical properties. Biological and physical properties of an organism are different sets of properties, but the physical ones determine the biological ones. Nonetheless, one can study only the biological properties and never deal with the physical ones.
The Korean-born philosopher Jaegwon Kim (“Concepts of Supervenience,” 1984) applied the concept to mind: mental properties are supervenient on physical (neural) properties. According to Kim, then, the mental is supervenient on the physical just like the macroscopic properties of objects supervene on their microscopic structures. Intuitively this means that mind is to brain what lightning is to electrically charged particles: the same phenomenon, that presents itself in two different ways.
Kim's supervenience defines a relationship between mental and physical, and it also defines some constraints. A mental state cannot correspond to two different physical states. Two brains can't be in the same mental state and be in different physical states. Mental states depend on corresponding neural states: any change in mental states must be matched by a corresponding change in physical states. Mental states "are" neural states, the same way that electricity "is" electrons.
One can organize nature in a hierarchy, starting with elementary particles and ending with consciousness. At each level some properties apply, but at the immediately higher level some other properties apply. For example, electrons have mass and spin, but electricity has potential and intensity. Chemical compounds have density and conductivity, whereas biological organisms have growth and reproduction. At each level a new set of properties "emerge": the weak force at the elementary particle level, viscosity at the molecular level, metabolism at the biological level, and so forth; and consciousness at the cognitive level.
The British philosopher Charles Dunbar Broad had already showed in the 1920s that the universe is inherently layered and that each layer yields the following layer but cannot explain the new properties that emerge with it.
Supervenience takes it for granted that nature works this way, but offers no explanation of why, at a higher level, we would find electricity instead of, say, “huicity” or “flowixity” (imaginary properties): why and how just those properties? Why and how does the mind emerge from the brain? Ultimately, this is the dilemma of "mental causation": how does the brain cause the mind? In general, this is the dilemma of "second-order properties": how do properties at one level cause properties at another level?
John Searle (who believes that minds are high-level features of brains) admits supervenience to the extent that it is causal: the same neural states are also the same mental states because the former cause the latter. Searle thus reduces supervenience to causality. But Kim does not impose any causal relationship: the relationship between the mental and the neural is analogous to the relationship between the usefulness of an object and the features that make it useful: those features do not "cause" its usefulness, they "constitute" its usefulness.
All facts of the universe depend (and are therefore supervenient) on physical facts, but the nature of such "dependence" is not trivial, according to the Australian philosopher David Chalmers. Properties that are supervenient on the physical world can normally be reduced to it (i.e., explained in terms of it), but consciousness is not truly, completely supervenient on the neural, and therefore it cannot be reduced to the neural. Consciousness is to some extent supervenient on the physical, but (by the nature of its kind of supervenience) it cannot be explained in physical terms.
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