" THERE WERE AT least two ways to solve any problem: from the beginning, which was the usual approach; and from the end, which was not. Likewise, every theorem could be proved either directly, using incremental logic, or indirectly, by conjecturing the negative of the hypothesis and demonstrating a contradiction. Thus there were at least four permutations to choose from. "

― Ethan Canin , A Doubter's Almanac