Attempts to do so have led to the strange world of "constructive mathematics''. The best way to explain indirect proofs is by showing you an example.
The presence of not's in the statement of a theorem we are trying to prove is often but not always!
Surprisingly little is known of his life, not even his dates or birthplace. Exercises 2. Shortly before BC, Ptolemy I founded the great university at Alexandria, the first institution of its kind, and not unlike the universities of today. Two famous stories are told about Euclid.
For example, the contrapositive of "If it is Sunday, I go to church'' is "If I am not going to church, it is not Sunday.