From the Beginning to Plato

350 PLATO: METAPHYSICS AND EPISTEMOLOGY
examples, philosophically important Forms were investigated, then the sort of
argumentation found in the Sophist passage might be required. 59
The Theaetetus and the Sophist
The Theaetetus represents Plato’s most sustained investigation into the nature of
knowledge. Structured around an attempt to define knowledge, it, like the early
dialogues, ends in frustration. But scholars have been quick to find positive
lessons which we are meant to draw from the discussion. 60
The dialogue divides into three sections which consider proposals to define
knowledge as (1) perception, (2) true judgement, and (3) true judgement with an
account. (3) resembles contemporary definitions of knowledge in terms of
justified true belief, but one difference is that all three suggestions define
knowledge not as a disposition but as a mental event: perception or judgement,
where judgement occurs when the soul says something to itself (180e–190a,
Sophist 265e–264a).
(1) Because knowledge is infallible and of what is, if knowledge is perception
then perception is infallible and of what is. Since x may (e.g.) appear warm to A
and cold to B, (1) entails a Protagorean relativism validating both perceptions:
what A perceives is for A and what B perceives is for B. The object is not warm
or cold in itself: no objective reality independent of the perceptions exists that
could falsify them.
Less straightforwardly, Plato connects (1) to a Heraclitean doctrine of constant
flux. If what a sensible object x is for A is nothing more than how x appears to
A, then x lacks an intrinsic nature—it is nothing in itself—and hence, in the
strong sense of ‘being’, x is nothing. Given the connection between ‘being’ and
permanence, x also lacks stability and constantly changes. For sensible objects
are continuously changing place so as to present different appearances to
different perceivers.
The refutation of (1) initially attacks the relativism and flux doctrines which it
is said to imply. Of several objections raised against Protagoras, the main
difficulty is that his position is self-refuting. Protagoras’ ‘Man is the measure’
doctrine was not that it appears to Protagoras that what appears to any person A
is for A, but that, absolutely, what appears to A is for A. But most people reject
this, that is, for most people it appears that it is not the case that what appears to
A is for A. So if what appears to them is for them, Protagoras’ doctrine does not
hold for them. Protagoras’ absolute claim is false.
As for the flux doctrine, if everything continuously changes in every respect,
no object can be accurately called anything since ‘it is always slipping away
while one is speaking’. So nothing we might call ‘perception’ is any more
perception than not perception, and therefore, on the proposed definition of
knowledge, nothing is any more knowledge than not knowledge. 61
Finally, Plato attacks the definition directly by arguing that no perception can
ever be an instance of knowledge. The argument has aroused much interest