This difference would perhaps be loosely called a difference of quality; but obviously it would be more accurate to call it a difference which consists in the fact that the one pattern has a constituent which is qualitatively different from any which the other has; and the difference between being qualitatively different and having qualitatively different constituents is important both because the latter can only be defined in terms of the former, and because it is possible for simple things to differ from one another in the former way, whereas it is only possible for complex things to differ in the latter.