In the Protagoras and the Gorgias, Plato shows Socrates in conversation with two famous Sophists.

The point is to bring the Socratic conception of the competency involved in living a good life into sharper focus by contrasting it with a competing one Plato associates with the Sophists. The Sophists appeared to many (and especially to the children of the aristocracy) to provide tools for living a good life, but Plato thought that this was an illusion and that the Sophists were harmful. The Sophists, as Plato portrays them, are salesman with no real interest in what Socrates describes in the Apology as "wisdom and truth and the best state of [the] soul."

Historical Sophists

Protagoras of Abdera. The most famous of the 5th century Sophists.
Gorgias of Leontini. The other famous 5th century Sophist. Younger contemporary of Protagoras. Famous also as an orator.
Prodicus of Ceos. Contemporary of Socrates.
Thrasymachus of Chalcedon. Last third of the 5th century. Plato features the character in conversation with Socrates in Book I of the Republic.
Hippias of Elis. Contemporary of Thrasymachus. Plato features the character in conversation with Socrates in the Hippias Major.

Associated Figures

Polus. Appears in conversation with Socrates in the Gorgias. Aristotle mentions him in the Metaphysics.
Callicles. Unknown outside Plato's dialogues and may be a character of Plato's invention. He appears in conversation with Socrates in the Gorigas.

Isocrates. 436 - 338 BCE. One of the ten attic orators. Has an interest in and view about φιλοσοφία.

The Rise of the Sophists

The Sophists were itinerant teachers of rhetoric primarily, but also of other subjects. Many of them came to Athens from other cities as part of political embassies (whose function was to negotiate on behalf of their home cites). Once in Athens, they stayed to fill a void in the traditional education system that had developed as Athens became powerful and her citizens became wealthy. In the law courts and elsewhere in Athenian society, the traditional education was no match for the ability in rhetorical persuasion the sophists taught.

"[I]f Hippocrates applies to me, he will learn precisely and solely that for which he has come. That learning consists of good judgment in his own affairs (εὐβουλία περὶ τῶν οἰκείων), showing how best to order his own home, and in the affairs of his city (πόλεως), showing how he may have most influence on public affairs both in speech and in action" (Protagoras 318e-319a).

Rhetorical persuasion appeared to many to be the key to desire satisfaction in both private and public matters and hence the key to the good life. In general, the sophists did little to disavow this impression. They were interested in making money, not in truth. They set themselves up as teachers, and the sons of the aristocracy paid huge sums of money to study with them.

  "Hippias, I can bear you witness that you speak the truth, and that your art (τέχνη) really has progressed in the direction of ability to carry on public together with private affairs. For this man Gorgias, the sophist from Leontini, came here from home in the public capacity of envoy, as being best able of all the citizens of Leontini to attend to the interests of the community, and it was the general opinion that he spoke excellently in the public assembly, and in his private capacity, by giving exhibitions and associating with the young, he earned and received a great deal of money from this city. Or, if you like, our friend here, Prodicus, often went to other places in a public capacity, and the last time, just lately, when he came here in a public capacity from Ceos, he gained great reputation by his speaking before the Council, and in his private capacity, by giving exhibitions and associating with the young, he received a marvelous sum of money. None of the ancients ever thought fit and right to exact the money as payment for his wisdom (σοφίας) or to give exhibitions among people of various places; so simple-minded were they, and so unconscious of the fact that money is of the greatest value. But either of these two has earned more money from his wisdom than any artisan from his art. And even before these Protagoras did the same.
  Why, Socrates, you know nothing of the beauties of this. For if you were to know how much money I have made, you would be amazed. I won't mention the rest, but once, when I went to Sicily, although Protagoras was staying there and had a great reputation and was the older, I, who was much younger, made in a very short time more than one hundred and fifty minas [= about what an unskilled worker could make in a lifetime], and in one very small place, Inycus [in southwest Sicily], more than twenty minas; and when I came home, I took this and gave it to my father, so that he and the other citizens were overwhelmed with amazement. And I pretty well think I have made more money than any other two sophists together" (Hippias Major 282b-e).

The Soul Requires Care

Socrates thought that living a good life requires a certain competency, that this competency is a matter of wisdom, and that this wisdom consists in knowledge of what is good and what is bad. As Plato understands them, the Sophists have a different conception of this competency. They think that it requires a competency, but they do not make knowledge of what is good and bad part of this competency. This is why Plato thought that the Sophists are so dangerous. He takes them to provide the means to get what one wants but not the knowledge of whether what one wants is good.

The dialogues suggest that Plato thought that the means to get what one wants, without knowledge of whether it is good, was in large measure the cause of Athen's political downfall. Plato's immediate audience lives after the great power and wealth of Periclean Athens has been completely dissipated in the Peloponnesian War. (With the exception of Gorgias, who had an unusually long life, the sophistical movement was dead when Plato wrote his dialogues.) This audience is nostalgic for the leadership of Pericles and life that had been lost. Plato does not share in this nostalgia. As he looks back, it seems to him that Pericles and the politicians failed Athens. They engaged in the practice the Sophists managed to package and sell. This practice was the problem, not Socrates, contrary to what many Athenians thought. This practice corrupts the young because it ruins the soul. To function properly, the soul requires the "care" Socrates urged. The political leaders did not provide it.

(Pericles (495-429 BCE) turned the Delian League into an Athenian Empire and also led Athens as general in the initial years of the Peloponnesian War (431-404 BCE). He promoted democracy, the arts, and public building projects, including the Parthenon, but the war was long and devastated Athens. Never again did the city regain its position of prominence in the ancient world.)

How rhetoric corrupts the soul

To explain how rhetoric corrupts the soul, Plato takes up the contrast between "reason" and "experience" that entered the philosophical tradition in the Presocratic Period. This occurs most clearly in two conversations in the Gorgias: with Polus in the explanation of what rhetoric is and with Callicles in the summary of the argument in the dialogue.

Socrates, in these two conversations, no longer just asks questions. He now presents views of his own. He says that rhetorical persuasion is flattery, that it is a matter of "experience," that this experience works without "reason," that it can move a human being to have goals he would not accept if his soul were healthy, and that it can ingrain a way of living that reason cannot easily challenge. The suggestion in these conversations is that "experience" is the ordinary, default way human beings get through life, that it is a way of living human beings share with animals, that human beings also have "reason," and that "reason" gives them the ability to take control over their lives, and that if human beings exercise "reason" in the right way, they will be "happy" (εὐδαίμων).

"[Y]ou will not easily find another, who, to use a rather absurd figure, attaches himself to the city as a gadfly to a horse, which, though large and well bred, is sluggish on account of his size and needs to be aroused by stinging. I think the god fastened me upon the city in some such capacity, and I go about arousing, and urging and reproaching each one of you, constantly alighting upon you everywhere the whole day long. Such another is not likely to come to you, gentlemen; but if you take my advice, you will spare me. But you, perhaps, might be angry, like people awakened from a nap, and might slap me, as Anytus advises, and easily kill me; then you would pass the rest of your lives in slumber, unless God, in his care for you, should send someone else to sting you" (Apology 30e-31a).

An empirical conception of expertise in rhetoric

In the Gorgias, in the context of whether Gorgias is skilled in an "art" (τέχνη), Plato has Polus say that expertise in general is a matter of "experience" (ἐμπειρία) and that rhetoric is an example. "[T]here are many arts (τέχναι) amongst mankind that have been discovered experimentally (ἐμπείρως), as the result of experiences (ἐμπειριῶν): for experience (ἐμπειρία) conducts the course of our life according to art, but inexperience according to chance. Of these several arts various men partake in various ways, and the best men of the best. Gorgias here is one of these..." (Gorgias 448c).

Gorgias' speech in defense of Helen suggests that he did try to work out an understanding of rhetorical persuasion, although it is not clear that he understands it in terms of "experience."

The "Encomium of Helen" is one Gorgias' two extant "epideictic" speeches. (The other is the "Defense of Palamedes.") Encomium is a Latin word deriving from the Greek ἐγκώμιον for laudatory ode. An ἐπίδειξις is an exhibition, display. In the speech, Gorgias argues that Helen of Troy is not to be blamed for the Trojan War despite the fact that her adultery and flight with Paris caused the War.

"For incantations divinely inspired by means of speeches are bringers of pleasure and removers of pain. For the power of an incantation, when it is conjoined with the opinion (δόξῃ) of the soul, beguiles it, persuades it, and transforms it by sorcery. For two arts have been discovered, those of sorcery and of magic, which are errors of the soul and deceptions of opinion. Whoever has persuaded, and also persuades, whomever about whatever does so by fabricating a false discourse. For if all men, with regard to all things, had memory of the ones that have passed by, understanding of the ones that are present, and forethought for the ones still to come, then a similar speech would not be similarly deceptive, as things are in fact at present, insofar as it is easy neither to remember what has passed by nor to examine what is present nor to divine what is to come. So that about most things most people furnish themselves with opinion as a counselor for the soul. But opinion, being slippery and unstable, involves those who use it in slippery and unstable successes. ... And as for the fact that persuasion, joining together with speech, also shapes the soul as it wishes: it is necessary to learn first the arguments of those who study the heavens, who, abolishing and establishing one opinion instead of another, have made things that are unbelievable and unclear appear to the eyes of opinion; second, contentions that constrain by means of speeches, in which one speech, written with artistry, not spoken with truth, delights and persuades a great crowd; third, contests of philosophical arguments, in which it is revealed that rapidity of thought too makes the conviction of an opinion easily changeable. The power of speech has the same relation with the arrangement of the soul as the arrangement of drugs has with the nature of bodies. For just as some drugs draw some fluids out of the body, and others other ones, and some stop an illness and others stop life, in the same way some speeches cause pain, others pleasure, others fear, others dispose listeners to courage, others drug and bewitch the soul by some evil persuasion" (Gorgias, "Encomium of Helen").

Perseus Digital Library:
Isocrates's, Against the Sophists, Antidosis
Henry George Liddell, Robert Scott, A Greek-English Lexicon:
ἐμπειρία, empeiria, noun, "experience,"
ρητορική, rētorikē, noun, "rhetoric,"
τέχνη, technē, noun, "art, skill"

Arizona State University Library. Loeb Classical Library: Early Greek Philosophy, Volume VIII: Sophists, Part 1