Three Elements of Reason

Firstly, there is the reception of the facts to reason about. These facts are received either from our own senses, or from the report of other minds; that is, either experience or authority supplies us with our material…
Secondly, there is the direct, simple act of the mind perceiving self-evident truth, as when we see that if A and B both equal C, then they are equal to each other. This act I call Intuition.
Thirdly, there is an art or skill of arranging the facts so as to yield a series of such intuitions which linked together produce a proof of the truth or falsehood of the propositions we are considering.

C.S. Lewis