An excerpt from Ketcham's "The Life of Abraham Lincoln: Entering the Law":
"In treating of this topic, it will be necessary to recall certain things already mentioned. One characteristic which distinguished Lincoln all through his life was thoroughness. When he was President a man called on him for a certain favor, and, when asked to state his case, made a great mess of it, for he had not sufficiently prepared himself. Then the President gave him some free advice. 'What you need is to be thorough,' and he brought his hand down on the table with the crash of a maul,–'to be thorough.' It was his own method. After a successful practise of twenty years he advised a young law student: 'Work, work, work is the main thing.' He spoke out of his own experience.
There is one remarkable passage in his life which is worth repeating here, since it gives an insight into the thoroughness of this man. The following is quoted from the Rev. J. P. Gulliver, then pastor of the Congregational church in Norwich, Conn. It was a part of a conversation which took place shortly after the Cooper Institute speech in 1860, and was printed in The Independent for September 1, 1864[:]
'Oh, yes! "I read law," as the phrase is; that is, I became a lawyer’s clerk in Springfield, and copied tedious documents, and picked up what I could of law in the intervals of other work. But your question reminds me of a bit of education I had, which I am bound in honesty to mention.
'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.”