The Civil War: A Narrative: Volume One: Fort Sumter to Perryville by Shelby Foote:
Though he did not believe at the outset that it would necessarily ever reach an end-—indeed, he believed it would not; otherwise it could never have done for him what it did—-this five-year “retreat,” coming as it did between his fortieth and his forty-fifth years, 1849 to 1854, was his interlude of greatest growth. Like many, perhaps most, men of genius, Lincoln developed late.
It was a time for study, a time for self-improvement. He went back and drilled his way through the first six books of Euclid, as an exercise to discipline his mind. Not politics but the law was his main interest now. Riding the circuit he talked less and listened more. Together with a new understanding and a deeper reading of Shakespeare and the Bible, this brought him a profounder faith in people, including those who had rejected him and repudiated what he had to offer as a leader. Here, too, he was learning. This was the period in which he was reported to have said, “You can fool some of the people all the time, and all the people some of the time, but you can’t fool all the people all the time.”