It is said of Cato that even from his infancy, in his speech, his countenance, and all his childish pastimes, he discovered an inflexible temper, unmoved by any passion, and firm in everything. He was resolute in his purposes, much beyond the strength of his age, to go through with whatever he undertook. He was rough and ungentle toward those that flattered him, and still more unyielding to those who threatened him. It was difficult to excite him to laughter, his countenance seldom relaxed even into a smile; he was not quickly or easily provoked to anger, but if once incensed, he was no less difficult to pacify.
When he began to learn, he proved dull, and slow to apprehend, but of what he once received, his memory was remarkably tenacious. And such in fact, we find generally to be the course of nature; men of fine genius are readily reminded of things, but those who receive with most pains and difficulty, remember best; every new thing they learn, being, as it were burnt and branded in on their minds. Cato's natural stubbornness and slowness to be persuaded may also have made it more difficult for him to be taught. For to learn is to submit to have something done to one; and persuasion comes soonest to those who have least strength to resist it.
-Plutarch's Lives, Translated by John Dryden, from the chapter, Cato The Younger