This article intends to show how the philosophy of Kant supposes a rupture with the doctrines based on a system of absolute rationality, where the most important element is the unity of reason. In this way, it will try to underline the main critiques of the Prussian philosopher to theories based on the unity of reason and direct access to reality, as well as exposing the Kantian proposal of a unitary formal rational structure, but with several irreconcilable uses.