Inverted Spectrum

Significance for Philosophy and Psychology

Behaviorism claimed a solution to the problem of knowing other minds—mental states are entirely evident in behavior. On one form, to be in mental state M is just to be disposed to exhibit behavior R in response to stimulus S. Thus, to sense red is to respond to long light wavelengths by saying “that's red” or “that's a ripe tomato.” But the IS raises an obvious problem: A purple sensation is on the face of it a different mental state than a red sensation is, yet an inverted spectrum would not affect overt behavior. Thus, behaviorism, which sought to reduce all mental states to response to stimuli, fails to account for qualitative mental states. Different states could cause the same behavior; the mental does not reduce entirely to stimulus-behavior links.

Functionalism and computationalism were successors to behaviorism developed starting in the 1960s. A functional account characterizes a mental state by its causal role: its inputs and outputs to other mental processes, in addition to the observable stimulus and response to which behaviorism was limited. For example, pain is that state that is typically caused by injury and that in turn might cause a mental planning module to avoid pressure on an injured limb. For a functionalist, a pain state is any state that has this causal role. But again, the inverted spectrum creates a problem: differing qualia states (red, blue) could have the same functional role in different individuals. Functionalism seeks to reduce each mental state to its causal role in a cognitive system, but the IS seems to show that different sensations could play the same causal role.

Indeed, perhaps normal behavior could be produced with no subjective consciousness at all. The IS suggests that the character of sensations doesn't matter and is undetectable; hence, an even more radical possibility than the IS is that some people have no subjective experiences at all—they behave the same in response to stimuli as normal conscious subjects but experience no qualia at all (while avowing that they do). This extension of the IS is the absent qualia or zombie possibility raised by Ned Block in the 1970s and widely discussed since.

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