for his
knowledge on others. But still this is no reason why he should always be
in doubt; of many personal, of many historical and scientific facts he
may be absolutely assured. And having such a mass of acknowledged truth
in the mathematical and physical, not to speak of the moral sciences,
the moderns have certainly no reason to acquiesce in the statement
that truth is appearance only, or that there is no difference between
appearance and truth.

The relativity of knowledge is a truism to us, but was a great
psychological discovery in the fifth century before Christ. Of this
discovery, the first distinct assertion is contained in the thesis of
Protagoras. Probably he had no intention either of denying or affirming
an objective standard of truth. He did not consider whether man in the
higher or man in the lower sense was a 'measure of all things.' Like
other great thinkers, he was absorbed with one idea, and that idea was
the absoluteness of perception. Like Socrates, he seemed to see that
philosophy must be brought back from 'nature' to 'truth,' from the world
to man. But he did not stop to analyze whether he meant 'man' in the
concrete or man in the abstract, any man or some men, 'quod semper quod
ubique' or individual private judgment. Such an analysis lay beyond
his sphere of thought; the age before Socrates had not arrived at these
distinctions. Like the Cynics, again, he discarded knowledge in any
higher sense than perception. For 'truer' or 'wiser' he substituted
the word 'better,' and is not unwilling to admit that both states and
individuals are capable of practical improvement. But this improvement
does not arise from intellectual enlightenment, nor yet from
the exertion of the will, but from a change of circumstances and
impressions; and he who can effect this change in himself or others may
be deemed a philosopher. In the mode of effecting it, while agreeing
with Socrates and the Cynics in the importance which he attaches to
practical life, he is at variance with both of them. To suppose that
practice can be divorced from speculation, or that we may do good
without caring about truth, is by no means singular, either in
philosophy or life. The singularity of this, as of some other
(so-called) sophistical doctrines, is the frankness with which they are
avowed, instead of being veiled, as in modern times, under ambiguous and
convenient phrases.

Plato appears to treat Protagoras much as he himself is treat