Fermat used his method of descent to prove that x^4 + y^4 = n^2 has no solution in positive integer values of x, y, and n, by showing that if there were a solution, there would be another with a smaller value of n.The history is pushed back a little more in this interesting note from Ed Sandifer to the Math History newsgroup, Victor Katz in his _A History of Mathematics_, pp 278 ff, describes