The idea of a third Critique based on fiction is suggested. For this, it is conjectured, on the one hand, that Kant resorts to the analogy of the as if in order to force the epistemic-aesthetic relationship of the subject with a presumed purpose of things, and, on the other, that he embeds the figure of the genius to solve the antinomy of a beautiful art according to purpose, but subject to rules. Said fiction would reflect the anxiety of a Kant who tries to show a unitary formal rational structure, but which consists of several irreconcilable uses.