This paper is about the ways in which psychologists categorise themselves into collectivities. It reports a study of the use of the categories 'mechanist' and 'humanist' in the transcript of an international psychology conference. The analysis documents: (1) the flexibility of category definition with respect to the subject matter, research methods and metaphysical assumptions of the putative membership; (2) the involvement of categories in practical tasks of criticism and evaluation; (3) some of the procedures used to warrant these categories as (...) 'natural'features of psychologists' social worlds. Problems for traditional historical, philosophical and sociological accounts of the development and nature of psychology raised by these findings are discussed and some of the pragmatic motivations for category discourse are assessed. (shrink)