solation of Being and the Megarian
or Cynic isolation of individuals are placed in the same class by Plato
(Soph.); and the same principle which is the symbol of motion to one
mind is the symbol of rest to another. The Atomists, who are sometimes
regarded as the Materialists of Plato, denied the reality of sensation.
And in the ancient as well as the modern world there were reactions from
theory to experience, from ideas to sense. This is a point of view from
which the philosophy of sensation presented great attraction to the
ancient thinker. Amid the conflict of ideas and the variety of opinions,
the impression of sense remained certain and uniform. Hardness,
softness, cold, heat, etc. are not absolutely the same to different
persons, but the art of measuring could at any rate reduce them all
to definite natures (Republic). Thus the doctrine that knowledge is
perception supplies or seems to supply a firm standing ground. Like the
other notions of the earlier Greek philosophy, it was held in a very
simple way, without much basis of reasoning, and without suggesting the
questions which naturally arise in our own minds on the same subject.

(b) The fixedness of impressions of sense furnishes a link of connexion
between ancient and modern philosophy. The modern thinker often repeats
the parallel axiom, 'All knowledge is experience.' He means to say that
the outward and not the inward is both the original source and the final
criterion of truth, because the outward can be observed and analyzed;
the inward is only known by external results, and is dimly perceived
by each man for himself. In what does this differ from the saying of
Theaetetus? Chiefly in this--that the modern term 'experience,' while
implying a point of departure in sense and a return to sense, also
includes all the processes of reasoning and imagination which have
intervened. The necessary connexion between them by no means affords a
measure of the relative degree of importance which is to be ascribed to
either element. For the inductive portion of any science may be small,
as in mathematics or ethics, compared with that which the mind has
attained by reasoning and reflection on a very few facts.

II. The saying that 'All knowledge is sensation' is identified by Plato
with the Protagorean thesis that 'Man is the measure of all things.'
The interpretation which Protagoras himself is supposed to give of these
latter words is: 'Things are to me as they appear t