People everywhere see red and green and yellow and blue. Still there are also people born congenitally blind everywhere; and there are tetrachromats, who see more colors than the rest of us, and people with various kinds of color blindness, who see fewer. In what sense, then, is it true that human color vision is universal?

(K. A. Appiah, Cosmopolitanism. Ethics in a World of Strangers, New York, W.W. Norton & Company, p. 94)