 There is little doubt that this is an important principle, one which plays a central part in the reasoning of many philosophers who have written on color, e.g., Wittgenstein 1977, Harrison 1973, Hardin 1988, Thompson 1995, Maund 1995. It is regarded as an important factor which physicalist-realist theories of color must explain, and have a problem in explaining. However, whatever the status of this principle, it is not a folk belief (and few of these last-mentioned theorists say that it is). Nor is it plausibly a tacit belief. For example, it does not seem a belief it is necessary to have, in order to have mastery of concepts of color and, in particular, of a concept of yellow. It is surely quite a sophisticated belief, which requires considerable experience with colors. For one thing, the dimensions mentioned by Johnston—hue, saturation and brightness—apply to aperture colors or film colors, which few folk would be aware of, and not to surface colors. Aperture colors are colors perceived under a special mode of viewing: one views the objects or light sources through a small aperture in a screen (of an achromatic color). The appearance of these colors differs from that of colors seen under more usual circumstances. “Surface colors” are the colors of illuminated samples seen under conditions in which it is possible for the viewer to distinguish the color of the surface from that of the ambient light. Indeed, for surface colors, there are two sets of dimensions: hue, chroma, and lightness (the Munsell system) and hue, chromaticness and whiteness/blackness (the Swedish Natural Color system, NCS). Nevertheless, Unity (or a set of Unities) is an important principle and it has something to do with our concepts of color. Wittgenstein, for example, thought it was central to our having the concepts of color that we do, but he says that “we do not want to find a theory of color … but rather the logic of color concepts” (Wittgenstein 1977:43e). (We are touching on deep issues of philosophical methodology, which we are not going to settle here; see the entry  analysis,  which, interestingly, discusses Jackson’s analysis of color.)