Letters 95.66

Its function is the same as that of precept. For he who utters precepts says: “If you would have self-control, act thus and so!” He who illustrates, says “The man who acts thus and so, and refrains from certain other things, possesses self-control.” If you ask what the difference here is, I say that the one gives the precepts of virtue, the other its embodiment. These illustrations, or, to use a commercial term, these samples, have, I confess, a certain utility; just put them up for exhibition well recommended, and you will find men to copy them.

Seneca, Letters to Lucilius 95.66, translated by Richard Mott Gummere