Theaetetus
seems to mean the generalized notion of feelings and impressions of
sense, without determining whether they are conscious or not.
The theory that 'Knowledge is sensible perception' is the antithesis of
that which derives knowledge from the mind (Theaet.), or which assumes
the existence of ideas independent of the mind (Parm.). Yet from their
extreme abstraction these theories do not represent the opposite poles
of thought in the same way that the corresponding differences would
in modern philosophy. The most ideal and the most sensational have a
tendency to pass into one another; Heracleitus, like his great successor
Hegel, has both aspects. The Eleatic isolation 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 to me, and to you as
they appear to you.' But there remains still an ambiguity both in the
text and in the explanation, which has to be cleared up. Did Protagoras
merely mean to assert the relativity of knowledge to the human mind? Or
did he mean to deny that there is an objective standard of truth?
These two questions have not been always clearly distinguished; the
relativity of knowledge has been sometimes confounded with uncertainty.
The untutored mind is apt to suppose that objects exist independently
of the human faculties, because they really exist independently of the
faculties of any individual. In the same way, knowledge appears to be
a body of truths stored up in books, which when once ascertained are
independent of the discoverer. Further consideration shows us that these
truths are not really independent of the mind; there is an adaptation of
one to the other, of the eye to the object of sense, of the mind to the
conception. There would be no world, if there neither were nor ever
had been any one to perceive the world. A slight effort of reflection
enables us to understand this; but no effort of reflection will enable
us to pass beyond the limits of our own faculties, or to imagine the
relation or adaptation of objects to the mind to be different from that
of which we have experience. There are certain laws of language and
logic to which we are compelled to conform, and to which our ideas
naturally adapt themselves; and we can no more get rid of them than we
can cease to be ourselves. The absolute and infinite, whether explained
as self-existence, or as the totality of human thought, or as the
Divine nature, if known to us at all, cannot escape from the category of
relation.
But because knowledge is subjective or relative to the mind, we are
not to suppose that we are therefore deprived of any of the tests or
criteria of truth. One man still remains wiser than another, a
more accurate observer and relater of facts, a truer measure of the
proportions of knowledge. The nature of testimony is not altered, nor
the verification of causes by prescribed methods less certain. Again,
the truth must often come to a man through others, according to the
measure of his capacity and education. But neither does this affect the
testimony, whether written or oral, which he knows by experience to
be trustworthy. He cannot escape from the laws of his own mind; and
he cannot escape from the further accident of being dependent 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 treated by
Aristotle; that is to say, he does not attempt to understand him from
his own point of view. But he entangles him in the meshes of a more
advanced logic. To which Protagoras is supposed to reply by Megarian
quibbles, which destroy logic, 'Not only man, but each man, and each
man at each moment.' In the arguments about sight and memory there is a
palpable unfairness which is worthy of the great 'brainless brothers,'
Euthydemus and Dionysodorus, and may be compared with the egkekalummenos
('obvelatus') of Eubulides. For he who sees with one eye only cannot be
truly said both to see and not to see; nor is memory, which is liable
to forget, the immediate knowledge to which Protagoras applies the
term. Theodorus justly charges Socrates with going beyond the truth;
and Protagoras has equally right on his side when he protests against
Socrates arguing from the common use of words, which 'the vulgar pervert
in all manner of ways.'
III. The theory of Protagoras is connected by Aristotle as well as Plato
with the flux of Heracleitus. But Aristotle is only following Plato,
and Plato, as we have already seen, did not mean to imply that such a
connexion was admitted by Protagoras himself. His metaphysical genius
saw or seemed to see a common tendency in them, just as the modern
historian of ancient philosophy might perceive a parallelism between
two thinkers of which they were probably unconscious themselves. We must
remember throughout that Plato is not speaking of Heracleitus, but of
the Heracliteans, who succeeded him; nor of the great original ideas of
the master, but of the Eristic into which they had degenerated a hundred
years later. There is nothing in the fragments of Heracleitus which at
all justifies Plato's account of him. His philosophy may be resolved
into two elements--first, change, secondly, law or measure pervading
the change: these he saw everywhere, and often expressed in strange
mythological symbols. But he has no analysis of sensible perception such
as Plato attributes to him; nor is there any reason to suppose that
he pushed his philosophy into that absolute negation in which
Heracliteanism was sunk in the age of Plato. He never said that 'change
means every sort of change;' and he expressly distinguished between
'the general and particular understanding.' Like a poet, he surveyed
the elements of mythology, nature, thought, which lay before him, and
sometimes by the light of genius he saw or seemed to see a mysterious
principle working behind them. But as has been the case with other great
philosophers, and with Plato and Aristotle themselves, what was really
permanent and original could not be understood by the next generation,
while a perverted logic carried out his chance expressions with an
illogical consistency. His simple and noble thoughts, like those of the
great Eleatic, soon degenerated into a mere strife of words. And when
thus reduced to mere words, they seem to have exercised a far wider
influence in the cities of Ionia (where the people 'were mad about
them') than in the life-time of Heracleitus--a phenomenon which, though
at first sight singular, is not without a parallel in the history of
philosophy and theology.
It is this perverted form of the Heraclitean philosophy which is
supposed to effect the final overthrow of Protagorean sensationalism.
For if all things are changing at every moment, in all sorts of ways,
then there is nothing fixed or defined at all, and therefore no sensible
perception, nor any true word by which that or anything else can be
described. Of course Protagoras would not have admitted the justice
of this argument any more than Heracleitus would have acknowledged the
'uneducated fanatics' who appealed to his writings. He might have said,