Aris asked:

We may both agree that something is red. But I cannot be sure if you see the same color I see. So we
may both stop our cars or come to the conclusion that someone is bleeding but do we see the same
thing? how can we know that there is such a thing as a ultimate nature of things even if we could
perceive it and agree about it?

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The short answer to your last sentence is: we can't. In fact the question as a whole touches on one of
the great pseudo-issues that are clobbered to death in the literature for no really good purpose. The
point is that our sense of vision does not deliver "absolute" colour values. There is a certain tolerance
of variation in the frequencies and chromatic hues in what you and I (assuming we have more or less
"normal" vision) would recognise as blood red or pretty much any other primary or secondary colour.
Disputes only arise in the finer shades in between those. There are two further considerations: (a)
that there is a very good evolutionary rationale for this supposed imprecision in our visual acuity,
namely: that we must detect the "same" colour (e.g. blood) under many different qualities of daylight,
room light, in semi darkness, under water etc. The second is that blood itself tend to have subtle
colour variations from one person to another. So in the end, if a colour detector were programmed to
deliver a "blood red" reading if it detected just one fixed "absolute" frequency, its success record
would be very much poorer than our own.

Jürgen Lawrenz

Sydney