I wrote once about Linconln's interest in the meaning of "demonstrate" and how it led him to study Euclid. I just came across a rather extended account of the same story posted by John Armstrong at The Unapologetic Mathematician which is worth repeating, so here 'tis:
"I want to point out something about Abraham Lincoln.
It’s said that he carried three books with him, particularly as a young itinerant lawyer. The Bible is a given. A collection of Shakespeare isn’t unexpected, considering his practiced ease with language. But the third? Lincoln studied Euclid, and carried a copy of the Elements. His law partner tells of Lincoln stretched out on the floor, reading geometry in the lamplight.
Lincoln himself explained his interest:
In the course of my law reading I constantly came upon the word “demonstrate”. I thought at first that I understood its meaning, but soon became satisfied that I did not. I said to myself, What do I do when I demonstrate more than when I reason or prove? How does demonstration differ from any other proof?
I consulted Webster’s Dictionary. They told of ‘certain proof,’ ‘proof beyond the possibility of doubt’; but I could form no idea of what sort of proof that was. I thought a great many things were proved beyond the possibility of doubt, without recourse to any such extraordinary process of reasoning as I understood demonstration to be. I consulted all the dictionaries and books of reference I could find, but with no better results. You might as well have defined blue to a blind man.
At last I said,- Lincoln, you never can make a lawyer if you do not understand what demonstrate means; and I left my situation in Springfield, went home to my father’s house, and stayed there till I could give any proposition in the six books of Euclid at sight. I then found out what demonstrate means, and went back to my law studies. "