ue perception in terms of the motion of atoms. Outer
objects were said to give off fine particles which, through the
mediation of the sense organs, impinged upon the soul-atom. But it was
evident even to the early exponents of this theory that according to
such an account, each perceiver is relegated to a world peculiar to his
own stand-point. His perception informs him concerning his own states as
affected by things, rather than concerning the things themselves. Upon
this ground the great sophist Protagoras is said to have based his
dictum: +Panto:n chre:mato:n metron anthro:pos+,--"Man is the measure of
all things." This is the classic statement of the doctrine of
relativity. But we have now entered into the province of epistemology,
and various alternatives confront us. Reduce thought to perception,
define perception as relative to each individual, and you arrive at
_scepticism_, or _the denial of the possibility of valid knowledge_.
Plato expounds this consequence in the well-known discussion of
Protagoras that occurs in the "Theaetetus."

"I am charmed with his doctrine, that what appears is to each
one, but I wonder that he did not begin his book on Truth with
a declaration that a pig or a dog-faced baboon, or some other
yet stranger monster which has sensation, is the measure of
all things; then he might have shown a magnificent contempt
for our opinion of him by informing us at the outset that
while we were reverencing him like a God for his wisdom, he
was no better than a tadpole, not to speak of his
fellow-men--would not this have produced an overpowering
effect? For if truth is only sensation, and no man can discern
another's feelings better than he, or has any superior right
to determine whether his opinion is true or false, but each,
as we have several times repeated, is to himself the sole
judge, and everything that he judges is true and right, why,
my friend, should Protagoras be preferred to the place of
wisdom and instruction, and deserve to be well paid, and we
poor ignoramuses have to go to him, if each one is the measure
of his own wisdom? . . . The attempt to supervise or refute
the notions or opinions of others would be a tedious and
enormous piece of folly, if to each man his own are right; and
this must be the case if Protagoras's Truth is the real truth,
and the philosopher is not merel