Reading or re-reading classics, you sometimes discover or re-discover thought-provoking sentences, such as:
"At present, the method which a programmer uses to convince himself of the correctness of his program is to try it out in particular cases and to modify it if the results produced do not correspond to his intentions. After he has found a reasonably wide variety of example cases on which the program seems to work, he believes that it will always work."
C. A. R. Hoare. An axiomatic basis for computer programming. Communications of the ACM, 12(10) :576–580, 1969.
Still, I love test-driven development.