Any dictionary will quickly confirm that most of the words which we recognise as straightforward and literal are dead (or "frozen") metaphor. Moreover if one attends carefully to the sentences of any fluent speaker one finds that they contain a steady stream of metaphors. The fluid boundaries of language surround us. Typically, however, the metaphors of ordinary discourse are transparent, so we pay little or no attention to the metaphorical character of ordinary discourse and the role that metaphor serves. However, while metaphor should be a central part of any inquiry which purports to provide a general explanation of language and communication, the important puzzles about language and meaning which metaphor raises are frequently treated as a peripheral issues in semantic theory, if they are mentioned at all. A central aim of this paper is to redress this neglect and to delineate the central role which metaphor plays in semantic evolution. Contemporary philosophers of language who are exceptions to the general pattern of neglect include Davidson (1978), whose deflationary view is that that there is no special category of metaphorical meaning distinct from literal meaning, and Searle (1979).