Abraham Lincoln was not an especially well-read man, but what he read he retained, thought about and frequently used. One author he was fond of was the Greek mathematician Euclid. His law partner Billy Herndon relates how Lincoln studied Euclid’s Elements:
He studied and nearly mastered the Six-books of Euclid (geometry) since he was a member of Congress. He began a course of rigid mental discipline with the intent to improve his faculties, especially his powers of logic and language. Hence his fondness for Euclid, which he carried with him on the circuit till he could demonstrate with ease all the propositions in the six books; often studying far into the night, with a candle near his pillow, while his fellow-lawyers, half a dozen in a room, filled the air with interminable snoring.
Lincoln wrote about why he decided to study Euclid:
In the fourth Lincoln Douglas debate Lincoln used Euclid to illustrate a point:
If you have ever studied geometry, you remember that by a course of reasoning, Euclid proves that all the angles in a triangle are equal to two right angles. Euclid has shown you how to work it out. Now, if you undertake to disprove that proposition, and to show that it is erroneous, would you prove it to be false by calling Euclid a liar?