s, also known as reductiones ad absurdum
s to the absurd
'') are quite common in mathematics. In fact, student
s 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.