The mind ranges to and fro, and spreads out, and advances forward with a quickness which has become a proverb, and a subtlety and versatility which baffle investigation. It passes on from point to point, gaining one by some indication; another on a probability; then availing itself of an association; then falling back on some received law; next seizing on testimony; then committing itself to some popular impression, or some inward instinct, or some obscure memory; and thus it makes progress not unlike a clamberer on a steep cliff, who, by quick eye, prompt hand, and firm foot, ascends how he knows not himself; by personal endowments and by practice, rather than by rule, leaving no track behind him, and unable to teach another. It is not too much to say that the stepping by which great geniuses scale the mountains of truth is as unsafe and precarious to men in general, as the ascent of a skilful mountaineer up a literal crag. It is a way which they alone can take; and its justification lies in their success. And such mainly is the way in which all men, gifted or not gifted, commonly reason,—not by rule, but by an inward faculty.
John Henry Newman, Oxford University Sermons, Sermon 13: Implicit and Explicit Reason.