We see from hence that when Aristotle comments upon the doctrine of Sokrates, what he here means is, the doctrine of the Platonic Sokrates in the Protagoras; the citation of this particular metaphor establishes the identity.
In another passage of the Nikom. Eth., Aristotle also cites a fact respecting the Sophist Protagoras, which fact is mentioned in the Platonic dialogue Protagoras — respecting the manner in which that Sophist allowed his pupils to assess their own fee for his teaching (Ethic. Nik. ix. 1, 1164, a. 25).
[104] Plato, Protag. p. 352 C. ἀλλ’ ἱκανὴν εἶναι τὴν φρόνησιν βοηθεῖν τῷ ἀνθρώπῳ.
Mistake of supposing that men act contrary to knowledge. We never call pleasures evils, except when they entail a preponderance of pain, or a disappointment of greater pleasures.
Sokr. — You speak well and truly. But you are aware that most men are of a different opinion. They affirm that many who know what is best, act against their own knowledge, overcome by pleasure or by pain. Prot. — Most men think so: incorrectly, in my judgment, as they say many other things besides.[105] Sokr. — When they say that a man, being overcome by food or drink or other temptations, will do things which he knows to be evil, we must ask them, On what ground do you call these things evil? Is it because they impart pleasure at the moment, or because they prepare disease, poverty, and other such things, for the future?[106] Most men would reply, I think, that they called these things evil not on account of the present pleasure which the things produced, but on account of their ulterior consequences — poverty and disease being both of them distressing? Prot. — Most men would say this. Sokr. — It would be admitted then that these things were evil for no other reason, than because they ended in pain and in privation of pleasure.[107] Prot. — Certainly. Sokr. — Again, when it is said that some good things are painful, such things are meant as gymnastic exercises, military expeditions, medical treatment. Now no one will say that these things are good because of the immediate suffering which they occasion, but because of the ulterior results of health, wealth, and security, which we obtain by them. Thus, these also are good for no other reason, than because they end in pleasures, or in relief or prevention of pain.[108] Or can you indicate any other end, to which men look when they call these matters evil? Prot. — No other end can be indicated.
[105] Plato, Protag. pp. 352-353.
[106] Plato, Protag. p. 353 D. πονηρὰ δὲ αὐτὰ πῇ φατε εἶναι; πότερον ὅτι τὴν ἡδονὴν ταύτην ἐν τῷ παραχρῆμα παρέχει καὶ ἡδύ ἐστιν ἕκαστον αὐτῶν, ἢ ὅτι εἰς τὸν ὕστερον χρόνον νόσους τε ποιεῖ καὶ πενίας καὶ ἄλλα τοιαῦτα πολλὰ παρασκευάζει;
[107] Plato, Protag. p. 353 E. Οὐκοῦν φαίνεται.… δι’ οὐδὲν ἄλλο ταῦτα κακὰ ὄντα, ἢ διότι εἰς ἀνίας τε ἀποτελευτᾷ καὶ ἄλλων ἡδονῶν ἀποστερεῖ;
[108] Plato, Protag. p. 354 B-C. Ταῦτα δὲ ἀγαθά ἐστι δι’ ἄλλο τι ἢ ὅτι εἰς ἡδονὰς ἀποτελευτᾷ καὶ λυπῶν ἀπαλλαγὰς καὶ ἀποτροπάς; ἢ ἔχετέ τι ἄλλο τέλος λέγειν, εἰς ὃ ἀποβλέψαντες αὐτὰ ἀγαθὰ καλεῖτε, ἀλλ’ ἢ ἡδονάς τε καὶ λύπας; οὐκ ἂν φαῖεν, ὡς ἐγᾦμαι.… Οὐκοῦν τὴν μὲν ἡδονὴν διώκετε ὡς ἀγαθὸν ὄν, τὴν δὲ λύπην φεύγετε ὡς κακόν;
Pleasure is the only good — pain the only evil. No man does evil voluntarily, knowing it to be evil. Difference between pleasures present and future — resolves itself into pleasure and pain.