The notion of a semantic field, the idea that both lexical items and the concepts associated with them come not singly but in groupings defined by specifiable relations of contrast and affinity, has had a limited but important influence among many who study language and thought. In particular, the concept has been important for some linguists, linguistic anthropologists, and semioticians. Among philosophers, the idea has scarcely been noticed.1 It has not often seemed to join with the sorts of concerns had by philosophers of language and mind. For one thing, semantic field theory is concerned with meaning and the subject of meaning has itself been on the defensive in the recent history of analytic philosophy of language, often relinquishing pride of place to a theory of reference. For another, semantic fields are about the meaning of words, and, where meaning is readmitted, it is sentence meaning that is at issue. Nonetheless, there have been a number of central problems in contemporary philosophy of language and mind that raise questions about word meaning in a way that semantic field theory can illuminate. Most notable of these are concerns about how we individuate psychological content, that is, about the conditions under which we assert the existence and identity of mental contents.