What is sex? Some feminists have harboured suspicions about this form of question, given its philosophical (or ‘metaphysical’1) pedigree. But philosophy no longer has the disciplinary monopoly on it. Indeed, with regard to sex, the more interesting task today is to pose and to attempt to answer the question from within a transdisciplinary problematic. For the question requires a theoretical response capable of recognizing that it concerns a cultural and political (and therefore neither a specifically philosophical nor a merely empirical) (...) problem. It requires an account of sex which is theoretically satisfying whilst being both adequate to and critical of everyday experience; a critical-theoretical account capable of embracing the everyday experience of sex, its lived contradictions. This article represents a first attempt to construct a transdisciplinary concept of sex to this end. It traces a line from Simone de Beauvoir’s The Second Sex to some recent attempts to define ‘sex’ and various related but importantly different concepts, and ends by proposing an answer to the question ‘What is sex?’ that draws on the philosophy of Immanuel Kant. For our transdisciplinary efforts will of necessity spring from some specific discipline(s) while not remaining confined within them, and not allowing them to remained confined within themselves (which has been something of a problem for philosophy, historically). (shrink)