Plato, Protagoras 350e-351b
“I do not here or anywhere else claim that the powerful are strong, but instead that the strong are powerful. For I believe that power and strength are not the same thing. One of them—power—comes from knowledge, or from madness, or anger; strength, however, comes from nature and the nourishing of the body.
So, for the first quality, daring and bravery are not the same thing. It can be the case that the brave are in fact daring, but the daring are not all brave. For boldness also comes to men from some type of skill or rage or madness, just like power, whereas bravery comes from nature and the nurturing of the mind.”
ἐγὼ δὲ οὐδαμοῦ οὐδ᾿ ἐνταῦθα ὁμολογῶ τοὺς δυνατοὺς ἰσχυροὺς εἶναι, τοὺς μέντοι ἰσχυροὺς δυνατούς· οὐ γὰρ ταὐτὸν εἶναι δύναμίν τε καὶ ἰσχύν, ἀλλὰ τὸ μὲν καὶ ἀπὸ ἐπιστήμης γίγνεσθαι, τὴν δύναμιν, καὶ ἀπὸ μανίας γε καὶ θυμοῦ, ἰσχὺν δὲ ἀπὸ φύσεως καὶ εὐτροφίας τῶν σωμάτων. οὕτω δὲ κἀκεῖ οὐ ταὐτὸν εἶναι θάρσος τε καὶ ἀνδρείαν· ὥστε συμβαίνει τοὺς μὲν ἀνδρείους θαρραλέους εἶναι, μὴ μέντοι τούς γε θαρραλέους ἀνδρείους πάντας· θάρσος μὲν γὰρ καὶ ἀπὸ τέχνης γίγνεται ἀνθρώποις καὶ ἀπὸ θυμοῦ γε καὶ ἀπὸ μανίας, ὥσπερ ἡ δύναμις, ἀνδρεία δὲ ἀπὸ φύσεως καὶ εὐτροφίας τῶν ψυχῶν γίγνεται.
It’s very difficult to arrive at a satisfactory translation for thymos. See, for example, this recent book on the thymos in Plato: http://bmcr.brynmawr.edu/2001/2001-04-13.html. The author speaks of thymos as a “living repository of Homeric values” which Plato strives to integrate into his own philosophical picture. Vishwa Adluri’s book Parmenides, Plato, and Mortal Philosophy: Return from Transcendence also has some very rich reflections on the concept, specifically in connection with the role it plays in Parmenides’ Proem. Thymos seems originally to have been one of the ways of speaking of the mortal soul as such, but which gradually became a faculty in an articulated concept of the soul, one that has to do with the sense of honor and shame and the whole web of emotions governed by them.
Erwin Cook, a homerist, insists that it should always be “temper” or emotionally based….I prefer a more neutral “heart”, as a Homerist too. But Erwin is pretty smart!
“Temper” used to be taken in a more general sense of one’s “temperament”, which would be somewhat appropriate, though probably with too much sense of composition, as opposed to a responsive faculty, but now “temper” has the connotation of “a tendency to become angry”, which narrows it in an unhelpful way. We just don’t seem to have a single adequate term in English corresponding to thymos, though I suppose “heart” comes closest.