The problems that students have with perceiving a need for proof are well known to all high-school teachers and have been identified without exception in all educational research as a major problem in the teaching of proof. Who has not yet experienced frustration when confronted by students asking "Why do we have to prove this?" Gonobolin (1954/1975) noted that "the students ... do not ... recognize the necessity of the logical proof of geometric theorems, especially when these proofs are of a visually obvious character or can easily be established empirically" (p. 61).