 For postmodernists, Nietzsche's genealogy of concepts in “On Truth and Lies in a Nonmoral Sense” (Nietzsche 1873, 77–97) is also an important reference. In this text, Nietzsche puts forward the hypothesis that scientific concepts are chains of metaphors hardened into accepted truths. On this account, metaphor begins when a nerve stimulus is copied as an image, which is then imitated in sound, giving rise, when repeated, to the word, which becomes a concept when the word is used to designate multiple instances of singular events. Conceptual metaphors are thus lies because they equate unequal things, just as the chain of metaphors moves from one level to another. Hegel's problem with the repetition of the “this” and the “now” is thus expanded to include the repetition of instances across discontinuous gaps between kinds and levels of things.