 In Chapters 1 and 2, to find a differential genetic principle, Deleuze works through the history of philosophy to isolate the concepts of “difference in itself” and “repetition for itself” that the assumptions of previous philosophies had prevented from being formulated. “Difference in itself” is difference that is freed from identities seen as metaphysically primary. Normally, difference is conceived of as an empirical relation between two terms which each has a prior identity of its own (“x is different from y”). Deleuze inverts this priority: identity persists, but is now a something produced by a prior relation between differentials (dx rather than not-x). Difference is no longer an empirical relation but becomes a transcendental principle that constitutes the sufficient reason of empirical diversity (for example, it is the difference of electrical potential between cloud and ground that constitutes the sufficient reason of the phenomenon of lightning).