John D. Norton
Department of History and Philosophy of Science
University of Pittsburgh

Einstein's early discoveries were made by thinking physically and
reflecting on special cases of clear physical meaning, as in his famous
use of thought experiments. He contrasted this method with one based on
formal or mathematical naturalness. In a remarkable notebook of private
calculations, written during his discovery of general relativity, we can
observe Einstein as he tested both approaches and failed to get them to
agree. In his later years Einstein turned to formal-mathematical methods
exclusively, proclaiming that "our experience hitherto justifies us in
believing that nature is the realization of the simplest conceivable
mathematical ideas."