Russell began with a puzzle inspired by the German philosopher Gottlob Frege’s (1848–1925) attempt to reduce mathematics to logic: Is the class of all classes that are not members of themselves, or C, itself a member of itself? This question seems valid, but Russell showed that it leads to contradictions: If C is a member of itself then it should not be in D, which is the class of classes that are not members of themselves, but if C is a member of itself, it will be in D. But if C is not a member of itself, then it should be in D, and C is a member of itself. Russell’s answer was that there is a hierarchy of types of things that restricts what can be said about them. So we can say that Russell is an analytic philosopher, but not that a group of people are an analytic philosopher.