The Path of Knowledge
An authoritative and accessible summary of Plato's Theaetetus   by Robert Cavalier (Carnegie Mellon University).

This article by Carol Poster is an unusually thorough treatment of this important philosopher. From The Internet Encyclopedia of Philosophy.

Protagoras (480-411 B.C.)
A concise and varied account of the philosopher.

Philosophical Background of the
Fifth Century B.C.

An excellent outline of the historical and political context and impact of the sophists.

The Philosophy
of the Sophists
A thoughtful article with related resources from the Radical Academy (which explicitly opposes relativism).

An informative article on the Sophists by George Briscoe Kerferd.

A detailed and accessible study of relativism in its various forms by Chris Swoyer (University of Oklahoma) from the Stanford Encyclopedia of Philosophy. It is long, but no longer than it need be to summarize this vital topic.




The Protagoras of Plato

Protagoras plays an important role in Plato's dialogues, one of which is named Protagoras and involves a fictional, yet realistic, conversation between the sophist and Socrates.  Another dialogue is named after a young boy named Theaetetus and involves a discussion between he, Socrates, and Theodorus who is a friend of Protagoras.  The dialogue begins with discussion about Protagoras' relativism, then it moves onto considerations about the nature of knowledge and closes with a definition of knowledge that has stayed with us over the millennia: Knowledge is justified, true belief.  That is the modern version of Plato's definition.  On this view, our beliefs will only count as knowledge when they are true (accord with the objective facts) and when the person who holds the belief has evidence or justification for it.

diagram of knowledge as justified true belief: perception is based on reality and causes belief; if the belief does not correspond with reality, then it is false and leads to error; if the belief does correspond with reality and has sufficient evidence, then it is knowledge.

In Plato's theory of knowledge (which he gives in Theaetetus as an alternative to Protagoras' theory), reality operates as a standard against which belief and perception can be measured.  Belief is created by perceptions of reality.  Note that Plato's theory of knowledge is also a theory of error, for it is possible to misperceive reality and create a false belief.  Also note that true belief alone is not the same as knowledge.  We could have a true belief by sheer luck.  In order to have genuine knowledge of reality, we must have both a true belief about it and sufficient justification (evidence) for that belief.  Plato acknowledges that the relations between justification and belief are not clear.  We may have no flawless way to tell when our evidence is sufficient.  The history of philosophy contains many efforts to correct this deficiency.  Scientific method is a system of building evidence by testing belief against observation (perception). 

Compare Plato's account to the Protagorean relativism which holds ; "Of all things the measure is Man, of the things that are, that they are, and of the things that are not, that they are not."  Plato interprets this claim as based on a theory, attributed to Protagoras, that holds knowledge and perception to be the same.  Plato points out that this implies that appearance and reality are indistinguishable and that no one can ever be mistaken about what they know.  When a person is in good health, a wine may taste sweet; but when she is ill, the same wine may taste bitter.  This the wine is neither sweet not bitter in itself, it only becomes sweet or bitter when related to an individual through perception.  In a way, there is no fixed reality at all.  There is only the reality becoming which is formed into nameable objects and qualities when acted on by perception.  You may be quite familiar with the idea that our minds create our reality.  This notion is popular in new age and self-help literature, magickal thought of the past and present, and even some contemporary versions of constructivist educational theory.  Below is a possible picture of Plato's interpretation of Protagoras' theory.

Compare this conception of knowledge to the Platonic theory depicted above. They are very different and even opposed in several regards. This diagram is based on the interpretation of Protagoras given in Theaetetus.  Read the following passage carefully by referring back to the diagram and checking to see whether the diagram accurately depicts what the passage describes.

"When the eye and the appropriate object meet together and give birth to whiteness and the sensation connatural with it, which could not have been given by either of them going elsewhere, then, while the sight: is flowing from the eye, whiteness proceeds from the object which combines in producing the colour; and so the eye is fulfilled with sight, and really sees, and becomes, not sight, but a seeing eye; and the object which combined to form the colour is fulfilled with whiteness, and becomes not whiteness but a white thing, whether wood or stone or whatever the object may be which happens to be coloured white. And this is true of all sensible objects, hard, warm, and the like, which are similarly to be regarded, as I was saying before, not as having any absolute existence, but as being all of them of whatever kind. generated by motion in their intercourse with one another; for of the agent and patient, as existing in separation, no trustworthy conception, as they say, can be formed, for the agent has no existence until united; with the patient, and the patient has no existence until united with the agent; and that which by uniting with something becomes an agent, by meeting with some other thing is converted into a patient. And from all these considerations, as I said at first, there arises a general reflection, that there is no one self-existent thing, but everything is becoming and in relation; and being must be altogether abolished, although from
habit and ignorance we are compelled even in this discussion to retain
the use of the term." Plato, Theaetetus,

This passage presents a remarkable conception of reality.  You can see in it why the Protagorean theory is called Relativism, because all knowledge and being itself is dependent upon relations between the perceiver and the perceived.  If this is the way things are, the implications are astounding.  Plato advances his criticism of relativism by drawing out a number of those implications.

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