Demetrius (1.5-1.6)

still, when men have led reckless lives, and have become conspicuous, in the exercise of power or in great undertakings, for badness, perhaps it will not be much amiss for me to introduce a pair or two of them into my biographies, though not that I may merely divert and amuse my readers by giving variety to my writing.

Ismenias the Theban used to exhibit both good and bad players to his pupils on the flute and say, you must play like this one, or again, you must not play like this one; and Antigenidas used to think that young men would listen with more pleasure to good flute-players if they were given an experience of bad ones also. So, I think, we also shall be more eager to observe and imitate the better lives if we are not left without narratives of the blameworthy and the bad.