of Plato's Theaetetus
Protagoras proposed that Man is
Knowledge is identical with
Basic formula: (I know x) = (I
Example: (I know I’m cold) = (I perceive that I am cold)
Perception is relative to the
perceiver. (I may feel cold when you feel hot)
Knowledge is subjective, but no
Argument that they are
“inseparable” from the perceiver
& only the perceiver can judge what is perceived.
No room for error in
judging own perception.
Therefore I know by perceiving.
Knowledge is always in flux (because perception is).
Objection based on memory:
If one remembers what was
known through perception, one knows whatever content it is that one
But (from above) perception
So when the perception is
finished, the knowledge is gone.
So in memory, one knows and
does not know the same content.
But this is impossible.
So knowledge can’t equal
Since the argument starts by
assuming that Protagoras is correct, then derives a bad result, this is a reductio
Objection from false opinion
Everyone thinks some
others wiser & some others less wise (e.g., we choose who should
guide us when we face trouble)
This behavior assumes a
difference between true and false opinion. (People at least perceive
others to be wiser or less wise.)
But to equate knowledge
with perception is to deny this distinction: if “man is the
measure” is true, then common practice (dismissing some people as
foolish) measures it as false. But if we say that the opinion of
others is false, then we deny that man is the measure. (For the
person who holds that “man is the measure” must grant that
others have true opinion, including about this.)
To avoid this paradox, we
should grant the people differ in their wisdom.
Argument that perception cannot
Each type of perception is
distinct from the rest (the various senses perceive “unconnected”
parts of the same object).
But to know that sounds
differ from colors cannot be an act of perception. (We know “both
are two” and the two are “unlike” each other)
“unlikeness” are universals, “unlike objects of sense” to
which they are applied.
To apply them requires a
”power” that is not a perception. (If one used only one’s
perception, one would “miss the truth” that they are unlike.)
Therefore we know something
that is not known by sensing/perceiving.
Therefore knowledge is
different from perception.