except as existing in some substance; and that nevertheless the same quality is thought to exist in many substances; green is a quality, which exists not by itself, but in grass and leaves of trees and so forth; at the same time, green may exist in many different leaves or blades of grass. The general principle which is thus illustrated in the quality green is readily understood to be true of all possible qualities; but unless we were able to think of some particular quality to illustrate it, we