We recognize the necessity of the principle but have no knowledge of the source of its necessity. We assume it solely in order to think its universality more determinately. The concepts of reality, substance, causality and necessity of existence have no meaning such as might serve to determine an object. They make possible the empirical knowledge of an object and explain the possibility of things in the world of sense. They cannot explain the possibility of the universe itself.
Pure reason is occupied with nothing but itself.
The unity of reason is the unity of system. It serves subjectively as a maxim that extends its application to all possible knowledge of objects. Furthers the extension of the empirical employment of the understanding, and, Guarantees its correctness. It is not a constitutive principle, it is merely regulative to further and strengthen in infinitum the empirical employment of reason. Reason cannot think this systematic unity otherwise than by giving to the idea of this unity an object.