The theory of behavior is useful to the life of man only as the index is useful to him who goes through it before reading the book itself; when he has read it, all that he has learned is the subject matter. Such is the moral teaching which we receive from the discourses, the precepts, and the stories we are treated to by those who bring us up. We listen to it all attentively; but when we have an opportunity to profit by the various advice we have been given, we become possessed by a desire to see if the thing will turn out to be what we have been told it will; we do it, and we are punished by repentance. What recompenses us a little is that in such moments we consider ourselves wise and hence entitled to teach others. Those whom we teach do exactly as we did, from which it follows that the world always stands still out goes from bad to worse.
Giacomo Casanova - History of my life (I, 3)
