Quote: Lewis on the Intellectus and Ratio

From The Discarded Image, by C. S. Lewis:

We are enjoying intellectus when we 'just see' a self-evident truth; we are exercising ratio when we proceed step by step to prove a truth which is not self-evident. A cognitive life in which all truth can be simply 'seen' would be the life of an intelligentsia, an angel. A life of unmitigated ratio where nothing was simply 'seen' and all had to be proved, would presumably be impossible; for nothing can be proved if nothing is self-evident. Man's mental life is spent in laboriously connecting those frequent, but momentary, flashes of intelligentsia which constitute intellectus. [Ch. 7: Sec. D]