In the late 60's, Nietzsche stamped, while clarifying his own epistemological bases, upon the same antagonisms that led his (neo-Kantian) paragons to the introduction of several purely hypothetical terms. In his essay On Truth and Lie we can find a lot of testimonies to the fact that Nietzsche used similar argumentation strategy. In my paper, I try to describe in detail the breaking point when Nietzsche was forced to incorporate into his theory (otherwise exact and supported by many scientific evidence) some irrational element in the form of a principle of power to generate metaphors.