Now applying this to colors, you may see that the point to which I am leading you is this. Just as the knife varies in value from other knives, so may one tint of black vary from another tint of black; one tint of red from another tint of red; one tint of yellow from another tint of yellow. Equally, since a certain quantity of crackers may have the same value as a certain quantity of cheese, so may a certain tint of red have the same value as a certain tint of yellow. But what is the standard by which one kind of color can be compared with another?