The Protagoras, like several of the Dialogues of Plato, is put into the mouth of Socrates, who describes a conversation which had taken place between himself and the great Sophist at the house of Callias. The main argument is between the elderly Protagoras, a celebrated Sophist, and Socrates. The discussion takes place at the home of Callias. A total of twenty-one people are named as present. Of the twenty-one people who are specifically said to be present, three are known Sophists. In addition to Protagoras himself, there are Hippias of Elis and Prodicus of Ceos. Two of the sons of Pericles are said to be there, Paralus and Xanthippus. With the exception of Aristophanes, all of Socrates' named friends from the Symposium are in attendance: Eryximachus the doctor, and Phaedrus are there, and so are the lovers Pausanias and Agathon (who is said to be a mere boy at this point), and Alcibiades. Additionally, there are several unnamed foreigners whom Protagoras is said to have picked up in his travels and a servant (a eunuch) in the employ of Callias. The dialogue begins with an unnamed friend of Socrates asking him how his pursuit of the young Alcibiades, just now reputed to be growing his first beard, was proceeding. Socrates explains that while he has just been in the company of Alcibiades, his mind is now on more interesting matters. He says that Protagoras, the wisest man alive, is in town. Socrates relates the story of how his young friend, Hippocrates, son of Apollodorus, came knocking on his door before daybreak and roused him out of bed. Hippocrates was in a big hurry to be present when Protagoras held court, as he was expected to do, at the home of Callias. Socrates warns the excitable Hippocrates that Sophists are dangerous. He tells him that the words of the Sophists go straight into the soul (psyche) and can corrupt a person straightaway. Socrates says that buying wisdom from a Sophist is different from buying food and drink at the market. With food and drink, you never know what you are getting, but you can consult experts for advice before consuming anything that might be dangerous In his dialogue, Protagoras, Plato credits him with having invented the role of the professional sophist. Protagoras also is believed to have created a major controversy during ancient times through his statement that, "Man is the measure of all things", interpreted by Plato to mean that there is no absolute truth, but that which individuals deem to be the truth. Although there is reason to question the extent of the interpretation of his arguments that has followed, that concept of individual relativity was revolutionary for the time, and contrasted with other philosophical doctrines that claimed the universe was based on something objective, outside human influence or perceptions.