The author introduces his article by comparing the reflections of Dorian Astor, a specialist on Nietzsche, on uncertainty, with those of the philosopher Charles Sanders Peirce. Astor makes an impassioned plea for uncertainty and denounces the harmful effects of certainty, two positions that can already be found in Peirce’s writings on belief. The author then points out that Peirce goes further in describing the dialectic between doubt and belief, since he generalizes it – thought has no other aim than to arrive at belief – and gives it an economic dimension, asserting that belief is a state of rest. In the third part, the author lays emphasis on the unconscious dimension of these psychic processes: he takes up the Project and then focuses on Guy Rosolato’s notion of the relation d’inconnu (the relation to the unknown), exploring the parallel between the two pairs of object relation/belief and relation to the unknown/doubt. The unknown has an anthropological dimension, that of the uncertain human being.