This paper examines the problem of natural kinds, a key problem within feminist theory, and argues for a non-instrumental realist account of group identity. I suggest that a reconstructed theory of essence helps to make sense of group membership because it combines a conventional account of groups with a realist commitment to there being something responsible for the appearance of regularities in the world. The claim that natural kind membership is a matter of similarity relationships manages to avoid metaphysical, universal (...) property-based essentialism and, at the same time, points to interested generative mechanisms underlying the appearance of the regularities that make sense of kind membership. Finally, I suggest that the concepts 'real essence' and 'internal constitution' offer a new way to conceive 'sex' and 'gender', and that Adorno's notion of non-identity is a way to think group membership non-instrumentally. (shrink)