Experiment #36 · Thought experiment

The Inverted Spectrum

Could your red be my green?

John Locke (precursor, 1689); modern: Sydney Shoemaker, Ned Block · 1689 / 1980s · Philosophy of mind

First published: Locke, *Essay Concerning Human Understanding* (1689), II.32; Shoemaker, "The Inverted Spectrum", *J. Phil.* 79 (1982).

Two people identical in every functional respect could, in principle, have systematically swapped colour qualia. Could we ever know?

Imagine two people, A and B, identical in every behavioural and functional respect — both call grass "green," stop at red lights, agree on which colours clash — but A's experience of red is what B has when seeing green, and vice versa. By hypothesis, no functional test could distinguish them. If such an inversion is coherent, qualitative consciousness is not fixed by functional organisation, and functionalism is in trouble. Defenders of functionalism (and many physicalists) argue the case is in fact incoherent at the relevant level of detail — colour space is asymmetric, so a clean swap is not possible. The case is one of the cleanest pressure-tests for whether consciousness is exhausted by causal-role structure.

Formulation

Two functionally indistinguishable subjects with systematically permuted colour experiences. Behavioural tests, neural tests, verbal reports: all identical. Yet by stipulation phenomenal character differs. If coherent: functionalism is incomplete.

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Dimensions Engaged

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Observer

Targets Observer · Physicality: does the qualitative character of experience reduce to functional/structural features, or is it a further fact that could vary while structure stays fixed?

Responses — How Schools Engage

Affirms / takes the bait 1

Dualism

A natural ally: qualia are properties over and above function. The conceivability of inversion is direct evidence that consciousness is not constituted by causal role.

Denies / rejects the premise 4

Naturalism

Functionalist naturalism: colour space is structurally asymmetric (yellow and blue are not symmetric to red and green); any actual inversion would produce detectable behavioural differences. The scenario is underspecified.

Structuralism

Phenomenal character is structural: it is exhausted by relations to other phenomenal states and to inputs and outputs. An "undetectable" inversion is a contradiction in terms.

Phenomenology

Phenomenology proceeds from how things appear, not from speculation about how they might differ behind identical appearances. The thought experiment trades on a third-person framing the discipline rejects.

Logical Positivism

An undetectable difference is, by the verification principle, no difference. The scenario is meaningless as stated.

Reframes the question 1

Panpsychism

Inversion may or may not be possible at the level of macro-experience, but the deeper question — what is the intrinsic nature of physical states — points to a panpsychist resolution.