Fabius used to say that the most shameful excuse a general could make was "I didn't think." I believe it's the most shameful excuse any person can make. Think about everything. Expect everything. Even good people will surprise you with something strange. Human nature creates minds that betray, feel no gratitude, want too much, and ignore what's sacred. When you're judging what one person might do, remember what people in general are capable of. When you're having the most fun, be most careful. When everything looks peaceful, remember that trouble isn't gone — it's just sleeping. Always believe something will happen to upset you. A ship's captain never spreads his sails with such confidence that he doesn't keep his gear ready to pull them down quickly.
Fabius was wont to say that the most shameful excuse a general could make was "I did not think." I think it the most shameful excuse that a man can make. Think of everything, expect everything: even with men of good character something queer will crop up; human nature produces minds that are treacherous, ungrateful, greedy, and impious: when you are considering what any man's morals may be, think what those of mankind are. When you are especially enjoying yourself, be especially on your guard: when everything seems to you to be peaceful, be sure that mischief is not absent, but only asleep. Always believe that something will occur to offend you. A pilot never spreads all his canvas abroad so confidently as not to keep his tackle for shortening sail ready for use.