[js1138]

If that specific shade of green can recur identically in more than one context, it's a universal in the sense in which ontologists use the word.

Perhaps I can give a concrete example.

It is possible to construct a set of color patches using different pigments, so that people with "normal" vision will say that two patches are identical, but people with color deficiencies will see them as different. Color blind people have been used to detect military camouflage, because they match colors differently.

Aside from monochromatic light, which can be described objectively as a wavelength, color is constructed in the eye and brain.

[OhioAttorney]

Good illustration. What it illustrates is one of two possible things: (a) two purportedly identical color-qualities may turn out on closer inspection not to be identical after all; or (b) two people may have slightly different subjective experiences under very similar external conditions, so that one person experiences the same (identical) shade twice where someone else experiences two different shades.

I see what you're getting at, but I think the illustration leaves open the possibility that each of the specific subjectively-experienced shades could be identically repeated.

Indeed, there's a sense in which each shade has to be 'repeated' even to be experienced at all: an instantaneous shade of color at a dimensionless point is no color at all, so the experience has to be 'spread out' over both space and time in order to happen in the first place. In that case, I can mentally subdivide either the space or the time and get two (or more) identical color-experiences.

Now I really have to go. Thanks to all of you for an interesting chat, and I expect I'll be around again on Monday or thereabouts.