definitions of science and knowledge. Proceeding from the lower to the
higher by three stages, in which perception, opinion, reasoning are
successively examined, we first get rid of the confusion of the idea of
knowledge and specific kinds of knowledge,--a confusion which has been
already noticed in the Lysis, Laches, Meno, and other dialogues. In the
infancy of logic, a form of thought has to be invented before the content
can be filled up. We cannot define knowledge until the nature of
definition has been ascertained. Having succeeded in making his meaning
plain, Socrates proceeds to analyze (1) the first definition which
Theaetetus proposes: 'Knowledge is sensible perception.' This is speedily
identified with the Protagorean saying, 'Man is the measure of all things;'
and of this again the foundation is discovered in the perpetual flux of
Heracleitus. The relativeness of sensation is then developed at length,
and for a moment the definition appears to be accepted. But soon the
Protagorean thesis is pronounced to be suicidal; for the adversaries of
Protagoras are as good a measure as he is, and they deny his doctrine. He
is then supposed to reply that the perception may be true at any given
instant. But the reply is in the end shown to be inconsistent with the
Heraclitean foundation, on which the doctrine has been affirmed to rest.
For if the Heraclitean flux is extended to every sort of change in every
instant of time, how can any thought or word be detained even for an
instant? Sensible perception, like everything else, is tumbling to pieces.
Nor can Protagoras himself maintain that one man is as good as another in
his knowledge of the future; and 'the expedient,' if not 'the just and
true,' belongs to the sphere of the future.

And so we must ask again, What is knowledge? The comparison of sensations
with one another implies a principle which is above sensation, and which
resides in the mind itself. We are thus led to look for knowledge in a
higher sphere, and accordingly Theaetetus, when again interrogated, replies