A gentle introduction to the basic idea: When is something equal to some other thing? http://www.math.harvard.edu/~mazur/preprints/when_is_one.pdf http://www.

A gentle introduction to the basic idea: When is something equal to some other thing? http://www.math.harvard.edu/~mazur/preprints/when_is_one.pdf http://www.math.harvard.edu/~mazur/preprints/when_is_one.pdf