Difference and Repetition is both the title of Gilles Deleuze's thesis and Tarde's fundamental principle. Invention produces differences; repetition allows for their diffusion; conflict is inevitable; no pre-established harmony allows for a solution: it is necessary to invent yet other solutions in order to temporarily generate other innovations, which, by repeating themselves, will produce other differences, and the cycle will begin again. That is the fundamental rhythm, the back beat that, alone, allows economic activity to acquire realism. What we need to follow in order to establish an economic science are "states of mind" and "logical duels."