I could swagger about Fortitude and the Contempt of Riches as much as Seneca himself.---Mandeville, Fable of the Bees (recall).
Far different is the purpose of those who are speech-making and trying to win the approbation of a throng of hearers, far different that of those who allure the ears of young men and idlers by many sided or fluent argumentation; philosophy teaches us to act, not to speak; it exacts of every one that he should live according to his own law [facere docet philosophia, non dicere, et hoc exigit, ut ad legem suam quisque vivat, ne orationi vita dissentiat vel ipsa inter se vita], that his life should not be out of harmony with his words, and that, further, there is unity to the color of one's actions [unus sit omnium actio num color]. This, I say, is the highest duty and the highest proof of wisdom, - that deed and word should be in accord, that a man should be equal to himself under all conditions, and always the same.--Seneca, Letter XX (slightly modified translation).
I am no stranger to falling short of my personal ideals (recall) and know plenty of well-established professional philosophers who are moralistic yet routinely exhibit lack of basic decency. I know extremely kind and generous folk who advocate hardship and worse for less articulate and connected others (in the name of utility, good governance, etc.). I know courageous advocates of civic liberties who fail their students by their incapacity at sublimating intellectual eros. So I did not expect it to find it so difficult to continue my series on Seneca's Letters and write about Letter XX in which Seneca addresses the discord between words and actions (and, thus, extends the theme of Letter XIX, recall). Given that Seneca is notorious for the mismatch between words and deeds, it is tempting to read this letter as Seneca's Apologia Pro Vita Sua addressing the known criticisms of his activities under Nero in which he grew rich in the service of a tyrant.
Upon extended reflection: the key to the letter is the implied distinction between being a teacher and being a philosopher (aka being wise and, explicitly, being great-souled [ingentis animi est]). Seneca identifies himself as a teacher not as a philosopher. He teaches, not unlike Plato [and unlike Nietzsche], in part, by pointing away toward proper exemplars (e.g., Demetrius the Cynic) and by drawing on Epicurean themes. Such exemplars have a three-fold function: (a) they offer an existence-proof that a comportment with integrity, that is, a harmony between right action and words, is humanly possible; (b) he holds up exemplars to be emulated (recall my treatment of Letter 5); (c) it allows him to deflect, in part, the charge of hypocrisy--for he very clearly implies that he falls short of the comportment with integrity while not being so corrupted (by the axiology of the marketplace/public [recall Letter 14]) that he cannot recognize true wisdom in others.
More important, from (c) it follows that (d) Seneca thinks he can bypass the more fundamental version of Meno's paradox (see Abe Stone); he can teach some virtue to Lucilius without being fully virtuous himself in a context where, as he implies, ordinary social institutions do not facilitate even such limited virtue. We may even add that Seneca shows some wisdom by recognizing his own limitations not only in light of exacting pure standards of integrity, but also in light of available human instantiations. (It is, thus, important that Demetrius is not merely a mythical or historical figure.)
So, Seneca knows he falls short of and he does not hold himself up as the standard for comportment with integrity. Thus the traditional charge against Seneca is (somewhat) misguided. Seneca may even hint that he knows intimately what he is warning against (do as I say, not as I did) and, thereby, (against Deleuze's protests) become a better teacher for some pupils (say, ambitious types like Lucilius). It is, after all, by no means obvious that Demetrius un-mediated by, say, a Seneca would be recognized as wise by an untutored pupil. Sincerity of even the noblest expressions does not guarantee moral approval, after all.
Ever since Socrates's trial we know that students' actions can reflect badly on the quality and reputation of their teachers' activities. Seneca embraces this fact--again, like Plato who shows us some of Socrates's more disastrous teaching methods (that's compatible with admiration of Socrates). So, it turns out, that Seneca is closer to the modern university professor than one might initially have thought. Like most of us, he is no holy man, and he does not expect to be evaluated as one.
Rather, Seneca correctly expects to be evaluated, in part, on the consequences of his teaching on his pupils (that is, the addressee and other, later readers of his Letters) and the societies their actions help shape. This is no eternal moral-get-out-of-jail-free-pass. Rather (at least late in life) Seneca embraces that he lives by a rather demanding standard that in addition to complex skills requires considerable luck (recall) and turns each of our teaching and writing lives into trials in which we are judged, fairly (with satire) or not, as causes of unknown (and potentially extended) multiplicity of effects (recall).* We may, then even say -- echoing Spinoza (Ethics 1p36 [recall]) -- that Seneca claims there is true glory [mea enim gloria erit] in being in virtue of such effects.