To know a man is to be able to detail the way his existence has unfolded, exactly. That is the theory : in practice, Rousseau only saves fragments, fragile remnants of a lost world. Exceptional negation communicates with the totality. The only opens up the all. Reflecting on the status of the autobiographical detail leads to an interrogation of the rapport between the art of Rousseau, which is one of selection, and his science of humankind, the approach of which is the opposite as it proceeds through summation.