From Plutarch’s Sympotic Questions or Table-Talk (as the Loeb puts it) 613F-614A. Translation is basically the Loeb, slightly revised.
In just such a manner a philosopher too, when with drinking-companions who are unwilling to listen to his homilies, will change his role, fall in with their mood, and not object to their activity so long as it does not transgress propriety. For he knows that, while men practise oratory only when they talk, they practise philosophy when they are silent, when they jest, even, by Zeus, when they are the butt of jokes and when they make fun of others. Indeed, not only is it true that ‘the worst injustice is to seem just when one is not,’ as Plato says, but also the height of understanding is to talk philosophy without seeming to do so, and by jests to accomplish the same as those who speak seriously. Just as the Maenads in Euripides, without shield and without sword, strike their attackers and wound them with their little thyrsoi, so true philosophers with their jokes and laughter somehow arouse men who are not altogether invulnerable and make them attentive.
οὕτω δὴ καὶ φιλόσοφος ἀνὴρ ἐν συμπόταις μὴ δεχομένοις τοὺς λόγους αὐτοῦ μεταθέμενος ἕψεται καὶ ἀγαπήσει τὴν ἐκείνων διατριβήν, ἐφ᾿ ὅσον μὴ ἐκβαίνει τὸ εὔσχημον, εἰδὼς ὅτι ῥητορεύουσι μὲν ἄνθρωποι διὰ λόγου, φιλοσοφοῦσι δὲ καὶ σιωπῶντες καὶ παίζοντες καὶ νὴ Δία σκωπτόμενοι καὶ σκώπτοντες. οὐ γὰρ μόνον ‘ἀδικίας ἐσχάτης ἐστίν,’ ὥς φησι Πλάτων, ‘μὴ ὄντα δίκαιον εἶναι δοκεῖν,’ ἀλλὰ καὶ συνέσεως ἄκρας φιλοσοφοῦντα μὴ δοκεῖν φιλοσοφεῖν καὶ παίζοντα διαπράττεσθαι τὰ τῶν σπουδαζόντων. ὡς γὰρ αἱ παρ᾿ Εὐριπίδῃ μαινάδες ἄνοπλοι καὶ ἀσίδηροι τοῖς θυρσαρίοις παίουσαι τοὺς ἐπιτιθεμένους τραυματίζουσιν, οὕτω τῶν ἀληθινῶν φιλοσόφων καὶ τὰ σκώμματα καὶ οἱ γέλωτες τοὺς μὴ παντελῶς ἀτρώτους κινοῦσιν ἁμωσγέπως καὶ συνεπιστρέφουσιν.