Monologium, on the Being of God66 Chapter LXVI. Through the rational mind is the nearest approach to the supreme Being.

Since it is clear, then, that nothing can be ascertained concerning this Nature in terms of its own peculiar character, but only in terms of something else, it is certain that a nearer approach toward knowledge of it is made through that which approaches it more nearly through likeness. For the more like to it anything among created beings is proved to be, the more excellent must that created being be by nature. Hence, this being, through its greater likeness, assists the investigating mind in the approach to supreme Truth; and through its more excellent created essence, teaches the more correctly what opinion the mind itself ought to form regarding the Creator. So, undoubtedly, a greater knowledge of the creative Being is attained, the more nearly the creature through which the investigation is made approaches that Being. For that every being, in so far as it exists, is like the supreme Being, reasons already considered do not permit us to doubt.

It is evident, then, that as the rational mind alone, among all created beings, is capable of rising to the investigation of this Being, so it is not the less this same rational mind alone, through which the mind itself can most successfully achieve the discovery of this same Being. For it has already been acknowledged that this approaches it most nearly, through likeness of natural essence. What is more obvious, then, than that the more earnestly the rational mind devotes itself to learning its own nature, the more effectively does it rise to the knowledge of that Being; and the more carelessly it contemplates itself, the farther does it descend from the contemplation of that Being?