But as soon as this is explained, another risk of confusion arises owing to the fact that when people contrast mere numerical difference with a kind of intrinsic difference, which is _not_ merely numerical, they are apt to identify the latter with _qualitative_ difference. It might, therefore, easily be thought that by 'difference in intrinsic nature' I mean 'difference in quality.' But this identification of difference in quality with difference in intrinsic nature would also be a mistake. It is true that what is commonly meant by difference of quality, in the strict sense, always is a difference of intrinsic nature: two things cannot differ in quality without differing in intrinsic nature; and that fact is one of the most important facts about qualitative difference. But the converse is by no means also true: although two things cannot differ in quality without differing in intrinsic nature, they can differ in intrinsic nature without differing in quality; or, in other words, difference in quality is only _one_ species of difference in intrinsic nature. That this is so follows from the fact that, as I explained, I am using the phrase 'different in intrinsic nature' as equivalent to 'not exactly like for it is quite plain that two things may not be exactly alike, in spite of the fact that they don't differ in quality, _e.g._ if the only difference between them were in respect of the _degree_ in which they possess some quality they do possess. Nobody would say that a very loud sound was exactly like a very soft one, even if they were exactly like in quality; and yet it is plain there is a sense in which their intrinsic nature is different For this reason alone qualitative difference cannot be identified with difference in intrinsic nature. And there are still other reasons. Difference in size, for instance may be a difference in intrinsic nature, in the sense I mean, but it can hardly be called a difference in quality. Or take such a difference as the difference between two patterns consisting in the fact that the one is a yellow circle with a red spot in the middle, and the other a yellow circle with a blue spot in the middle. 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.