nucleation

LIT-1348: Two geometrical arguments against similarity structuralism

2 min read

Properties5
StatusActive — read in full 2026-10-22 (NOTE-1055) from the publisher's open-access PDF (Springer, CC BY 4.0); worth reading as the objection to similarity structuralism that does not rest on intuition: it takes accepted facts of colour science, that exactly four hues look unmixed and every other hue a mixture of two, and that the black–white axis is a single gradient where the axes through grey are opponent pairs, and argues that no adequate feature of the colour similarity space's geometry marks either. Its decisive premise is argued by inspecting CIELAB and extrapolating to better models, not by an analysis, and its target is similarity structuralism about colour only.
Version2
Tagsconsciousness, metaphysics
Cited as “paper” byNOTE-1055 — Aldé — Two geometrical arguments against similarity structuralism
Cited as “source” byTHEORY-867 — The geometry of the colour similarity space does not mark the four unique hues, nor the difference between the black–white axis, along which every grey is a mixture of both ends, and the opponent axes, along which each half shows only one end; so a structuralism on which phenomenal character is fixed by similarity relations alone cannot account for how colour experiences are composed

Graph View

Aldé, D. (2026). Two geometrical arguments against similarity structuralism. Synthese 207(5): 204. doi:10.1007/s11229-026-05562-5.

Key takeaways

From the reading, NOTE-1055.

  • The target. “The phenomenal character of a mental quality is fully determined by its similarity relations with the other phenomenal characters in the similarity space” (p. 4). Rosenthal’s discrimination spaces and structuralisms using other relations are set aside.
  • What structuralism implies. Similarity spaces “must be asymmetric”, since two symmetric points “should feel the same, hence they should be the same point” (p. 5).
  • Unique hues. Exactly four hues are experienced as unmixed, and no geometric property of the colour solid singles them out (§4.1).
  • Opposing dimensions. “only the black-white axis is so peculiar in its phenomenology, and it is not equally peculiar in its geometrical uniqueness” (p. 15). This argument does not need the unique hues.
  • Introspection. Structuralism is built on introspected similarity, so it cannot dismiss introspected composition without specific reasons (§5).
  • The suggested alternative is to ask how far opponent-process models “might inform a metaphysics of mental qualities” (p. 18).

Why this is here

The owner’s exchange of 2026-10-21, “Experience and interpretation”, asked whether experience is constituted by interpretation or produced by it. The exchange cited this paper as an objection to similarity structuralism: similarity relations do not fix phenomenal character. It is an objection to that view, not to every structural view, and the exchange took it to limit how far the thesis can rest on similarity alone. Bears on CLAIM-412.

The reading bears this description out, with one narrowing: the argument is made for colour experience only, and the paper is silent on other modalities. It also sets a test the claim has not met. Whatever organization fixes colour character must yield the unique hues and the opponent structure, which a similarity space, on Aldé’s argument, does not.

Filed from this work on 2026-10-22: THEORY-867, on why the geometry of colour similarity cannot account for the composition of colour experience.

Backlinks