  It might appear that types are better construed as sets (assuming sets themselves are not universals). The natural thought is that a type is the set of its tokens, which is how Quine sometimes (1987, p. 218) construes a type. After all, a species is often said to be “the class of its members”. There are two serious problems with this construal. One is that many types have no tokens and yet they are different types. For example, there are a lot of very long sentences that have no tokens. So if a type were just the set of its tokens, these distinct sentences would be wrongly classified as identical, because each would be identical to the null set. Another closely related problem also stems from the fact that sets, or classes, are defined extensionally, in terms of their members. The set of natural numbers without the number 17 is a distinct set from the set of natural numbers. One way to put this is that classes have their members essentially. Not so the species homo sapiens, the word ‘the’, nor Beethoven's Symphony No. 9. The set of specimens of homo sapiens without George W. Bush is a different set from the set of specimens of homo sapiens with him, but the species would be the same even if George W. Bush did not exist. That is, it is false that had George W. Bush never existed, the species homo sapiens would not have existed. The same species might have had different members; it does not depend for its existence on the existence of all its members as sets do.