Diogenes Laertius' book Lives of the Philosophers is source for many early Greek philosophers including Protagoras; but this work was compiled over six hundred years after Protagoras' death and consequently not accurate. Plato also wrote about Protagoras, considering him a sophist; Aristotle regarded Protagoras as who first introduced a spiritual principle, as a sober man among the inebriated.
Protagoras' doctrines can be divided into three groups:
- Orthoepeia: the study of the correct use of words
- Man-measure statement: the notion that knowledge is relative to the knower
- Agnosticism: the claim that we cannot know anything about the gods. 
"Dialog" by Plato.
Is not that true, Protagoras? What else would you say?
He assented, but with great reluctance. Then temperance and wisdom are the same, as before justice and holiness appeared to us to be nearly the same. And now, Protagoras, I said, we must finish the enquiry, and not faint. Do you think that an unjust man can be temperate in his injustice?
I should be ashamed, Socrates, he said, to acknowledge this which nevertheless many may be found to assert.
And shall I argue with them or with you? I replied.
I would rather, he said, that you should argue with the many first, if you will.
Whichever you please, if you will only answer me and say whether you are of their opinion or not. My object is to test the validity of the argument; and yet the result may be that I who ask and you who answer may both be put on our trial.
Protagoras at first made a show of refusing, as he said that the argument was not encouraging; at length, he consented to answer.
Now then, I said, begin at the beginning and answer me. You think that some men are temperate, and yet unjust?
Yes, he said; let that be admitted.
And temperance is good sense?
And good sense is good counsel in doing injustice?
If they succeed, I said, or if they do not succeed?
If they succeed.
And you would admit the existence of goods?
And is the good that which is expedient for man?
Yes, indeed, he said: and there are some things which may be inexpedient, and yet I call them good.
I thought that Protagoras was getting ruffled and excited; he seemed to be setting himself in an attitude of war. Seeing this, I minded my business, and gently said:-
When you say, Protagoras, that things inexpedient are good, do you mean inexpedient for man only, or inexpedient altogether? and do you call the latter good?
Certainly not the last, he replied; for I know of many things-meats, drinks, medicines, and ten thousand other things, which are inexpedient for man, and some which are expedient; and some which are neither expedient nor inexpedient for man, but only for horses; and some for oxen only, and some for dogs; and some for no animals, but only for trees; and some for the roots of trees and not for their branches, as for example, manure, which is a good thing when laid about the roots of a tree, but utterly destructive if thrown upon the shoots and young branches; or I may instance olive oil, which is mischievous to all plants, and generally most injurious to the hair of every animal with the exception of man, but beneficial to human hair and to the human body generally; and even in this application (so various and changeable is the nature of the benefit), that which is the greatest good to the outward parts of a man, is a very great evil to his inward parts: and for this reason physicians always forbid their patients the use of oil in their food, except in very small quantities, just enough to extinguish the disagreeable sensation of smell in meats and sauces.
When he had given this answer, the company cheered him. And I said: Protagoras, I have a wretched memory, and when any one makes a long speech to me I never remember what he is talking about. As then, if I had been deaf, and you were going to converse with me, you would have had to raise your voice; so now, having such a bad memory, I will ask you to cut your answers shorter, if you would take me with you. cf: Protagoras - Dialogues by Plato .
- Protagoras By Plato The Internet Classics Archive.