One of the first things Feser tackles is the problem of universals. This philosophical problem has a 2000 year history, and is still a matter of debate today. Yet Feser claims that conceptualism and nominalism, the major alternatives to Feser's preferred realism, are demonstrably false. I guess none of those other philosophers were smart enough to see that the problem had already been solved.
As examples of universal, Feser lists "triangularity," "humanness," and "redness." And by realism about them, he means that they "exist objectively, apart from the human mind and distinct from any material or physical features of the world."
Now, redness is an interesting choice. We now know a lot more about color perception than Aristotle did. What we know as colors are the result of a complex interaction among the light source, the surface of the colored object, the physiology of the eye, and the processing that goes on in the brain.
Start with an obvious point: "red" is an English word. We can translate it into other languages - but can we be sure that the extent of "rot" in German (for instance) coincides with "red" in English? Maybe, and maybe not. But some languages don't have a word that even vaguely resembles "red." The Jale of New Guinea have only two color terms: roughly, "brilliant" and "dull." The Tiv of Nigeria have three color terms:
ii, which encompasses all greens, some blues, and some greys; pupu, which encompasses very light blues, light greys, and white; and nyian, which encompasses red, some browns, orange and yellow.When someone says "The apple is red (nyian)" in Tiv, they are saying something very different than I am when I say that sentence. So it is clear that color is a culturally dependent quantity.
Even within our own culture, people can differ about what a term means. "That's not red, that's maroon!" If twenty people looked at this chart and picked out the colors they would label "red," there might not be two lists that agree. And it's not just a matter of definition: people's perceptions of color can differ dramatically. This is most obvious in color blindness, but here's a personal example that doesn't involve color blindness. When my wife and I were house hunting, we discussed one house that had (my wife thought) beautiful green marble counter-tops in the kitchen. "They weren't green, they were black!" I exclaimed. And it wasn't just a matter of lighting or recollection: we now live in that house, and my wife maintains that we have green counter-tops (with black flecks), while I am sure that they are black (with some green in them).
But if you really want proof that colors are not mind-independent, just look at the following image:
Do you see the green and blue spirals? So do I: but there aren't any green and blue spirals. Those spirals are exactly the same color. (r = 0, g = 255, b = 150) That is to say, in every objective, measurable sense, they are the same. Yet our minds interpret them as different colors.
Imagine that an extraterrestrial lands on Earth. Its physiology is completely different from ours. Maybe it doesn't even perceive light: it gets around by some sort of sonar. How would you explain why an apple and a fire engine are both red? How would you explain why two of those spirals are green and the other two blue?
It is clear, then, that colors are not mind-independent. But that at least leaves open the possibility that other things that seem obviously mind-independent are, in fact, mind-dependent.