There are some subtleties there. Andrej Bauer covers them well: http://math.andrej.com/2010/03/29/proof-of-negation-and-proof-by-contradiction/ http://math.andr

There are some subtleties there. Andrej Bauer covers them well: http://math.andrej.com/2010/03/29/proof-of-negation-and-proof-by-contradiction/ http://math.andrej.com/2010/03/29/proof-of-negation-and-proo...