Indirect

proofs, also known as

*reductiones ad absurdum* (``

reductions to the

absurd'') are quite common in mathematics. In fact,

students of

mathematics tend to use them

too often---providing an indirect proof where a

direct one would do just as well.

Intuitionist mathematics (which is much more well-grounded than the name would seem to indicate; it is related to constructivism) does not allow the indirect method of mathematical proof.