Laudator Temporis Acti

Plutarch, How a Man May Become Aware of His Progress in Virtue 15 = Moralia 85a-b (tr. Frank Cole Babbitt, with his note):

With men of this sort it has already become a constant practice, on proceeding to any business, or on taking office, or on encountering any dispensation of Fortune, to set before their eyes good men of the present or of the past,^a and to reflect: "What would Plato have done in this case? What would Epameinondas have said? How would Lycurgus have conducted himself, or Agesilaus?" And before such mirrors as these, figuratively speaking, they array themselves or readjust their habit, and either repress some of their more ignoble utterances, or resist the onset of some emotion.

^aSeneca (Epistulae Moral. ad Lucilium, i.11.8) says that this idea comes from Epicurus.

ἤδη δὲ τοῖς τοιούτοις παρέπεται τὸ βαδίζουσιν ἐπὶ πράξεις τινὰς ἢ λαβοῦσιν ἀρχὴν ἢ χρησαμένοις τύχῃ τίθεσθαι πρὸ ὀφθαλμῶν τοὺς ὄντας ἀγαθοὺς ἢ γενομένους, καὶ διανοεῖσθαι "τί δ᾿ ἂν ἔπραξεν ἐν τούτῳ Πλάτων, τί δ᾿ ἂν εἶπεν Ἐπαμεινώνδας, ποῖος δ᾿ ἂν ὤφθη Λυκοῦργος ἢ Ἀγησίλαος," οἷόν τι πρὸς ἔσοπτρα κοσμοῦντας ἑαυτοὺς ἢ μεταρρυθμίζοντας ἢ φωνῆς ἀγεννεστέρας αὑτῶν ἐπιλαμβανομένους ἢ πρός τι πάθος ἀντιβαίνοντας.

Seneca, Letters to Lucilius 1.11.8-10 (tr. Richard M. Gummere, with his note):

But my letter calls for its closing sentence. Hear and take to heart this useful and wholesome motto^a: "Cherish some man of high character, and keep him ever before your eyes, living as if he were watching you, and ordering all your actions as if he beheld them." Such, my dear Lucilius, is the counsel of Epicurus; he has quite properly given us a guardian and an attendant. We can get rid of most sins, if we have a witness who stands near us when we are likely to go wrong. The soul should have someone whom it can respect,—one by whose authority it may make even its inner shrine more hallowed. Happy is the man who can make others better, not merely when he is in their company, but even when he is in their thoughts! And happy also is he who can so revere a man as to calm and regulate himself by calling him to mind! One who can so revere another, will soon be himself worthy of reverence. Choose therefore a Cato; or, if Cato seems too severe a model, choose some Laelius, a gentler spirit. Choose a master whose life, conversation, and soul-expressing face have satisfied you; picture him always to yourself as your protector or your pattern. For we must indeed have someone according to whom we may regulate our characters; you can never straighten that which is crooked unless you use a ruler.

^a Epicurus, Frag. 210 Usener.

Iam clausulam epistula poscit. Accipe, et quidem utilem ac salutarem, quam te affigere animo volo: "aliquis vir bonus nobis diligendus est ac semper ante oculos habendus, ut sic tamquam illo spectante vivamus et omnia tamquam illo vidente faciamus." Hoc, mi Lucili, Epicurus praecepit; custodem nobis et paedagogum dedit, nec immerito. Magna pars peccatorum tollitur, si peccaturis testis assistit. Aliquem habeat animus quem vereatur, cuius auctoritate etiam secretum suum sanctius faciat. O felicem illum qui non praesens tantum sed etiam cogitatus emendat! O felicem qui sic aliquem vereri potest ut ad memoriam quoque eius se conponat atque ordinet! Qui sic aliquem vereri potest cito erit verendus. Elige itaque Catonem; si hic tibi videtur nimis rigidus, elige remissioris animi virum Laelium. Elige eum cuius tibi placuit et vita et oratio et ipse animum ante se ferens vultus; illum tibi semper ostende vel custodem vel exemplum. Opus est, inquam, aliquo ad quem mores nostri se ipsi exigant: nisi ad regulam prava non corriges.

Related post: WWJD.