To say that languages are systems is to draw a line around what we are studying. ‘How much is systematic?’ points out that a line drawn differently might include much that is less determinate. For instance, if differences in pitch or meaning cannot be reduced to exact contrasts, they are strictly not part of one's knowledge of the language as a system. Many linguists search for constraints that reveal the character of language, or of languages, in general. Therein lies the excitement of Chomsky's Universal Grammar. If he is right, it is reasonable to concentrate on what belongs to it, and abstract away from interfering factors that do not.