What Lincoln took from Euclid was the idea that, if you were careful, you could erect a tall, rock-solid building of belief and agreement by rigorous deductive steps, story by story, on a foundation of axioms no one could doubt: or, if you like, truths one holds to be self-evident. Whoever doesn’t hold those truths to be self-evident is excluded from discussion. I hear the echoes of Euclid in Lincoln’s most famous speech, the Gettysburg Address, where he characterizes the United States as “dedicated to the proposition that all men are created equal.” A “proposition” is the term Euclid uses for a fact that follows logically from the self-evident axioms, one you simply cannot rationally deny.