If one were to crystallise twentieth-century continental philosophy into a single problem, it would be the ‘problem of difference’. The demand to think a concept of difference and differences that is not subordinate to a primary conception of identity, but would be constitutive of those given identities, is evident in Heidegger's ‘ontico-ontological difference’ and in his later ‘dif-ferenz’, Merleau-Ponty's chiasmatic notion of ‘flesh’, Levinas's ‘face of the other’, Foucault's ‘thought of the outside’, Irigaray's ‘sexual difference’, Butler's ‘gender performativity’, and Lyotard's ‘differend’. These point to the ubiquitous sense in twentieth-century continental thought that, across all domains – ontological, ethical, social, political, and so on – our efforts to think the nature of things will always be shortcircuited by the same self-enclosed, representational categories by which we attempt to think them in the first place. When confined to only what things have in common, thought cannot get at the heart of what makes them singular, what John Duns Scotus called ‘haecceity’, or the ‘thisness’ of a thing. To truly think the nature of the thing, thought must reach to the constitutive conditions of those identities, and to the differences and relations between those identities.

The ‘problem of difference’ is that difference and relation necessarily elude the stasis of representational thought, which traditionally seeks to fix borders around conceptual content, thereby halting the passages between various concepts. In order to truly think difference, then, it must be conceptualised on its own terms, constrained neither by the logic of identity, nor, consequently, by the requirements of a standard philosophical concept. It must not be thought as a merely empirical relation between given things, nor should it be conceived in the Hegelian manner as a diametrically opposed contradiction which, by virtue of its bipolar and reciprocal nature, would ultimately unite dialectically into a higher, homeostatic identity. The ‘philosophy of difference’ is the designation for the philosophical response to this problem.