“Euclid’s first common notion is this:  ‘Things which are equal to the same thing are equal to each other.’   That’s a rule of mathematical reasoning.  It’s true because it works.  Has done and always will do.  In his book, Euclid says this is self-evident.  You see, there it is.  Even in that 2,000 year old book of mechanical law, it is a self-evident truth that things which are equal to the same thing are equal to each other.”

–Lincoln quoting Euclid in the new movie “Lincoln”