Deleuze’s main philosophical project in his early works (i.e., those prior to his collaborations with Guattari) can be baldly summarized as a systematic inversion of the traditional metaphysical relationship between identity and difference. Traditionally, difference is seen as derivative from identity: e.g., to say that “X is different from Y” assumes some X and Y with at least relatively stable identities. To the contrary, Deleuze claims that all identities are effects of difference. Identities are neither logically nor metaphysically prior to difference, Deleuze argues, “given that there exist differences of nature between things of the same genus.”[14] That is, not only are no two things ever the same, the categories we use to identify individuals in the first place derive from differences. Apparent identities such as “X” are composed of endless series of differences, where “X” = “the difference between x and x’”, and “x” = “the difference between…”, and so forth. Difference goes all the way down. To confront reality honestly, Deleuze claims, we must grasp beings exactly as they are, and concepts of identity (forms, categories, resemblances, unities of apperception, predicates, etc.) fail to attain difference in itself. “If philosophy has a positive and direct relation to things, it is only insofar as philosophy claims to grasp the thing itself, according to what it is, in its difference from everything it is not, in other words, in its internal difference.”[15]

via Gilles Deleuze – Wikipedia, the free encyclopedia.