Ref. : In my blog 'Mathematical approach for
daily problem'
Professor of mathematics in Stanford
University.
He worked on a great variety of mathematical topics, including
series, number theory, combinatorics, and probability. In his later
days, he spent considerable effort on trying to characterize the
general methods that people use to solve problems, and to describe
how problem-solving should be taught and learned (source : Wikipedia, George Pólya). In his book "How to solve it",
he gave general strategy to solve given mathematical problem.
Personally, I call him as 'Pólya Sensei (Dear Teacher Pólya)' in my
mind, though I have never met him. 
Book : How to Solve It: A New Aspect of Mathematical
Method,
new ed., 2004 (Japanese version is here).