ancient Greek philosopher Plato. Plato was Socrates' student and
Aristotle's teacher. As in most of Plato's dialogues, the main
character is Socrates. In the Theaetetus, Socrates converses with
Theaetetus, a boy, and Theodorus, his mathematics teacher. Although
this dialogue features Plato's most sustained discussion on the
concept of knowledge, it fails to yield an adequate definition of
knowledge, thus ending inconclusively. Despite this lack of a positive
definition, the Theaetetus has been the source of endless scholarly
fascination. In addition to its main emphasis on the nature of
cognition, it considers a wide variety of philosophical issues: the
Socratic Dialectic, Heraclitean Flux, Protagorean Relativism,
rhetorical versus philosophical life, and false judgment. These issues
are also discussed in other Platonic dialogues.
The Theaetetus poses a special difficulty for Plato scholars trying to
interpret the dialogue: in light of Plato's metaphysical and
epistemological commitments, expounded in earlier dialogues such as
the Republic, the Forms are the only suitable objects of knowledge,
and yet the Theaetetus fails explicitly to acknowledge them. Might
this failure mean that Plato has lost faith in the Forms, as the
Parmenides suggests, or is this omission of the Forms a calculated
move on Plato's part to show that knowledge is indeed indefinable
without a proper acknowledgement of the Forms? Scholars have also been
puzzled by the picture of the philosopher painted by Socrates in the
digression: there the philosopher emerges as a man indifferent to the
affairs of the city and concerned solely with "becoming as much
godlike as possible." What does this version of the philosophic life
have to do with a city-bound Socrates whose chief concern was to
benefit his fellow citizens? These are only two of the questions that
have preoccupied Plato scholars in their attempt to interpret this
highly complex dialogue.
1. The Characters of Plato's Theaetetus
In the Theaetetus, Socrates converses with two mathematicians,
Theaetetus and Theodorus. Theaetetus is portrayed as a physically ugly
but extraordinarily astute boy, and Theodorus is his mathematics
teacher. According to the Oxford Classical Dictionary, Theaetetus
lived in Athens (c. 415–369 BCE) and was a renowned geometer. He is
credited with the theory of irrational lines, a contribution of
fundamental importance for Euclid's Elements X. He also worked out
constructions of the regular solids like those in Elements XIII.
Theodorus lived in Cyrene in the late fifth century BCE. In the
dialogue, he is portrayed as a friend of Protagoras, well-aware of the
Sophist's teachings, and quite unfamiliar with the intricacies of
Socratic Dialectic. As far as his scientific work is concerned, the
only existing source is Plato's Theaetetus: In the dialogue, Theodorus
is portrayed as having shown the irrationality of the square roots of
3, 5, 6, 7, … ,17. Irrational numbers are those that are not equal to
any fraction having whole numbers in the numerator and denominator.
The passage has been interpreted in many different ways, and its
historical accuracy has been disputed.
2. Date of Composition
The introduction of the dialogue informs the reader that Theaetetus is
being carried home dying of wounds and dysentery after a battle near
Corinth. There are two known battles that are possibly the one
referred to in the dialogue: the first one took place at about 394
BCE, and the other occurred at around 369 BCE. Scholars commonly
prefer the battle of 369 BCE as the battle referred to in the
dialogue. The dialogue is a tribute to Theaetetus' memory and was
probably written shortly after his death, which most scholars date
around 369 – 367 BCE. It is uncontroversial that the Theaetetus, the
Sophist and the Statesman were written in that order. The primary
evidence for this order is that the Sophist begins with a reference
back to the Theaetetus and a reference forward to the Statesman. In
addition, there is a number of thematic continuities between the
Theaetetus and the Sophist (for instance, the concept of "false
belief," and the notions of "being," "sameness," and "difference") and
between the Sophist and the Statesman (such as the use of the method
of "collection and division").
3. Outline of the Dialogue
The dialogue examines the question, "What is knowledge (episteme)?"
For heuristic purposes, it can be divided into four sections, in which
a different answer to this question is examined: (i) Knowledge is the
various arts and sciences; (ii) Knowledge is perception; (iii)
Knowledge is true judgment; and (iv) Knowledge is true judgment with
an "account" (Logos). The dialogue itself is prefaced by a
conversation between Terpsion and Euclid, in the latter's house in
Megara. From this conversation we learn about Theaetetus' wounds and
impending death and about Socrates' prophecy regarding the future of
the young man. In addition, we learn about the dialogue's recording
method: Euclid had heard the entire conversation from Socrates, he
then wrote down his memoirs of the conversation, while checking the
details with Socrates on subsequent visits to Athens. Euclid's role
did not consist simply in writing down Socrates' memorized version of
the actual dialogue; he also chose to cast it in direct dialogue, as
opposed to narrative form, leaving out such connecting sentences as
"and I said" and "he agreed." Finally, Euclid's product is read for
him and for Terpsion by a slave. This is the only Platonic dialogue
which is being read by a slave.
a. Knowledge as Arts and Sciences (146c – 151d)
To Socrates' question, "What is knowledge?," Theaetetus responds by
giving a list of examples of knowledge, namely geometry, astronomy,
harmonics, and arithmetic, as well as the crafts or skills (technai)
of cobbling and so on (146c–d). These he calls "knowledges,"
presumably thinking of them as the various branches of knowledge. As
Socrates correctly observes, Theaetetus' answer provides a list of
instances of things of which there is knowledge. Socrates states three
complaints against this response: (a) what he is interested in is the
one thing common to all the various examples of knowledge, not a
multiplicity of different kinds of knowledge; (b) Theaetetus' response
is circular, because even if one knows that, say, cobbling is
"knowledge of how to make shoes," one cannot know what cobbling is,
unless one knows what knowledge is; (c) The youth's answer is
needlessly long-winded, a short formula is all that is required. The
definition of clay as "earth mixed with water," which is also evoked
by Aristotle in Topics 127a, is representative of the type of
definition needed here. Theaetetus offers the following mathematical
example to show that he understands Socrates' definitional
requirements: the geometrical equivalents of what are now called
"surds" could be grouped in one class and given a single name
("powers") by dint of their common characteristic of irrationality or
incommensurability. When he tries to apply the same method to the
question about knowledge, however, Theaetetus does not know how to
proceed. In a justly celebrated image, Socrates, like an intellectual
midwife, undertakes to assist him in giving birth to his ideas and in
judging whether or not they are legitimate children. Socrates has the
ability to determine who is mentally pregnant, by knowing how to use
"medicine" and "incantations" to induce mental labor. Socrates also
has the ability to tell in whose company a young man may benefit
academically. This latter skill is not one that ordinary midwives seem
to have, but Socrates insists that they are the most reliable
matchmakers, and in order to prove his assertion he draws upon an
agricultural analogy: just as the farmer not only tends and harvests
the fruits of the earth, but also knows which kind of earth is best
for planting various kinds of seed, so the midwife's art should
include a knowledge of both "sowing" and "harvesting." But unlike
common midwives, Socrates' art deals with the soul and enables him to
distinguish and embrace true beliefs rather than false beliefs. By
combining the technê of the midwife with that of the farmer, Socrates
provides in the Theaetetus the most celebrated analogy for his own
philosophical practice.
b. Knowledge as Perception (151d – 186e)
Encouraged by Socrates' maieutic intervention, Theaetetus comes up
with a serious proposal for a definition: knowledge is perception.
Satisfied with at least the form of this definition, Socrates
immediately converts it into Protagoras' homo-mensura doctrine, "Man
is the measure of all things, of the things that are that [or how]
they are, of the things that are not that [or how] they are not." The
Protagorean thesis underscores the alleged fact that perception is not
only an infallible but also the sole form of cognition, thereby
bringing out the implicit assumptions of Theaetetus' general
definition. Socrates effects the complete identity between knowledge
and perception by bringing together two theses: (a) the interpretation
of Protagoras' doctrine as meaning "how things appear to an individual
is how they are for that individual" (e.g., "if the wind appears cold
to X, then it is cold for X"); and (b) the equivalence of "Y appears F
to X" with "X perceives Y as F" (e.g., "the wind appears cold to
Socrates" with "Socrates perceives the wind as cold"). His next move
is to build the ontological foundation of a world that guarantees
perceptual infallibility. For that, Socrates turns to the Heraclitean
postulate of Radical Flux, which he attributes to Protagoras as his
Secret Doctrine. Nearly all commentators acknowledge that Protagoras'
secret teaching is unlikely to be a historically accurate
representation of either Protagoras' ontological commitments or
Heraclitus' Flux doctrine. The notion of Universal Flux makes every
visual event—for example the visual perception of whiteness—the
private and unique product of interaction between an individual's eyes
and an external motion. Later this privacy is explained with the
metaphor of the perceiver and the perceived object as parents birthing
a twin offspring, the object's whiteness and the subject's
corresponding perception of it. Both parents and offspring are unique
and unrepeatable: there can be no other, identical interaction between
either the same parents or different parents able to produce the same
offspring. No two perceptions can thus ever be in conflict with each
other, and no one can ever refute anyone else's perceptual judgments,
since these are the products of instantaneous perceptual relations,
obtaining between ever-changing perceiving subjects and ever-changing
perceived objects. Although the assimilation of Protagorean Relativism
to Theaetetus' definition requires the application of the doctrine to
Perceptual Relativism—which explains Socrates' extensive focus on the
mechanics of perception—one should bear in mind that the
man-as-measure thesis is broader in scope, encompassing all judgments,
especially judgments concerning values, such as "the just" and "the
good," and not just narrowly sensory impressions. Socrates launches a
critique against both interpretations of Protagoreanism, beginning
with its broad—moral and epistemological—dimensions, and concluding
with its narrow, perceptual aspects.
Socrates attacks broad Protagoreanism from within the standpoint
afforded him by three main arguments. First, Socrates asks how, if
people are each a measure of their own truth, some, among whom is
Protagoras himself, can be wiser than others. The same argument
appears in Cratylus 385e–386d as a sufficient refutation of the
homo-mensura doctrine. The Sophists' imagined answer evinces a new
conceptualization of wisdom: the wisdom of a teacher like Protagoras
has nothing to do with truth, instead it lies in the fact that he can
better the way things appear to other people, just as the expert
doctor makes the patient feel well by making his food taste sweet
rather than bitter, the farmer restores health to sickly plants by
making them feel better, and the educator "changes a worse state into
a better state" by means of words (167a).
The second critique of Protagoras is the famous self-refutation
argument. It is essentially a two-pronged argument: the first part
revolves around false beliefs, while the second part, which builds on
the findings of the first, threatens the validity of the
man-as-measure doctrine. The former can be sketched as follows: (1)
many people believe that there are false beliefs; therefore, (2) if
all beliefs are true, there are [per (1)] false beliefs; (3) if not
all beliefs are true, there are false beliefs; (4) therefore, either
way, there are false beliefs (169d–170c). The existence of false
beliefs is inconsistent with the homo-mensura doctrine, and hence, if
there are false beliefs, Protagoras' "truth" is false. But since the
homo-mensura doctrine proclaims that all beliefs are true, if there
are false beliefs, then the doctrine is manifestly untenable. The
latter part of Socrates' second critique is much bolder—being called
by Socrates "the most subtle argument"—as it aims to undermine
Protagoras' own commitment to relativism from within the relativist
framework itself (170e–171c). At the beginning of this critique
Socrates asserts that, according to the doctrine under attack, if you
believe something to be the case but thousands disagree with you about
it, that thing is true for you but false for the thousands. Then he
wonders what the case for Protagoras himself is. If not even he
believed that man is the measure, and the many did not either (as
indeed they do not), this "truth" that he wrote about is true for no
one. If, on the other hand, he himself believed it, but the masses do
not agree, the extent to which those who do not think so exceed those
who do, to that same extent it is not so more than it is so.
Subsequently, Socrates adds his "most subtle" point: Protagoras
agrees, regarding his own view, that the opinion of those who think he
is wrong is true, since he agrees that everybody believes things that
are so. On the basis of this, he would have to agree that his own view
is false. On the other hand, the others do not agree that they are
wrong, and Protagoras is bound to agree, on the basis of his own
doctrine, that their belief is true. The conclusion, Socrates states,
inevitably undermines the validity of the Protagorean thesis: if
Protagoras' opponents think that their disbelief in the homo-mensura
doctrine is true and Protagoras himself must grant the veracity of
that belief, then the truth of the Protagorean theory is disputed by
everyone, including Protagoras himself.
In the famous digression (172a–177c), which separates the second from
the third argument against broad Protagoreanism, Socrates sets up a
dichotomy between the judicial and the philosophical realm: those
thought of as worldly experts in issues of justice are blind followers
of legal practicalities, while the philosophical mind, being
unrestricted by temporal or spatial limitations, is free to
investigate the true essence of justice. Civic justice is concerned
with the here-and-now and presupposes a mechanical absorption of rules
and regulations, whereas philosophical examination leads to an
understanding of justice as an absolute, non-relativistic value. This
dichotomy between temporal and a-temporal justice rests on a more
fundamental conceptual opposition between a civic morality and a
godlike distancing from civic preoccupations. Godlikeness, Socrates
contends, requires a certain degree of withdrawal from earthly affairs
and an attempt to emulate divine intelligence and morality. The
otherworldliness of the digression has attracted the attention of,
among others, Aristotle, in Nicomachean Ethics X 7, and Plotinus, who
in Enneads I 2, offers an extended commentary of the text.
In his third argument against broad Protagoreanism, Socrates exposes
the flawed nature of Protagoras' definition of expertise, as a skill
that points out what is beneficial, by contrasting sensible
properties—such as hot, which may indeed be immune to interpersonal
correction—and values, like the good and the beneficial, whose essence
is independent from individual appearances. The reason for this,
Socrates argues, is that the content of value-judgments is properly
assessed by reference to how things will turn out in the future.
Experts are thus people who have the capacity to foresee the future
effects of present causes. One may be an infallible judge of whether
one is hot now, but only the expert physician is able accurately to
tell today whether one will be feverish tomorrow. Thus the predictive
powers of expertise cast the last blow on the moral and
epistemological dimensions of Protagorean Relativism.
In order to attack narrow Protagoreanism, which fully identifies
knowledge with perception, Socrates proposes to investigate the
doctrine's Heraclitean underpinnings. The question he now poses is:
how radical does the Flux to which the Heracliteans are committed to
must be in order for the definition of knowledge as perception to
emerge as coherent and plausible? His answer is that the nature of
Flux that sanctions Theaetetus' account must be very radical, indeed
too radical for the definition itself to be either expressible or
defensible. As we saw earlier, the Secret Doctrine postulated two
kinds of motion: the parents of the perceptual event undergo
qualitative change, while its twin offspring undergoes locomotive
change. To the question whether the Heracliteans will grant that
everything undergoes both kinds of change, Socrates replies in the
affirmative because, were that not the case, both change and stability
would be observed in the Heraclitean world of Flux. If then everything
is characterized by all kinds of change at all times, what can we say
about anything? The answer is "nothing" because the referents of our
discourse would be constantly shifting, and thus we would be deprived
of the ability to formulate any words at all about anything.
Consequently, Theaetetus' identification of knowledge with perception
is deeply problematic because no single act can properly be called
"perception" rather than "non perception," and the definiendum is left
with no definiens.
After Socrates has shown that narrow Protagoreanism, from within the
ontological framework of radical Heracliteanism, is untenable, he
proceeds to reveal the inherent faultiness of Theaetetus' definition
of knowledge as perception. In his final and most decisive argument,
Socrates makes the point that perhaps the most basic thought one can
have about two perceptible things, say a color and a sound, is that
they both "are." This kind of thought goes beyond the capacity of any
one sense: sight cannot assess the "being" of sound, nor can hearing
assess that of color. Among these "common" categories, i.e.,
categories to which no single sensual organ can afford access,
Socrates includes "same," "different," "one," and "two," but also
values, such as "fair" and "foul." All of these are ascertained by the
soul through its own resources, with no recourse to the senses.
Theaetetus adds that the soul "seems to be making a calculation within
itself of past and present in relation to future" (186b). This remark
ties in with Socrates' earlier attribution to expertise of the ability
to predict the future outcome of present occurrences. But it also
transcends that assertion in the sense that now a single unified
entity, the soul, is given cognitive supremacy, in some cases with the
assistance of the senses whereas in other cases the soul "itself by
itself." Perception is thus shown to be an inadequate candidate for
knowledge, and the discussion needs to foreground the activity of the
soul when "it is busying itself over the things-which-are" (187a). The
name of that activity is judging, and it is to this that the second
part of the conversation now turns.
c. Knowledge as True Judgment (187a – 201c)
While true judgment, as the definiens of knowledge, is the ostensible
topic of the discussants' new round of conversation, the de facto
topic turns out to be false judgment. Judgment, as the soul's internal
reasoning function, is introduced into the discussion at this
juncture, which leads Theaetetus to the formulation of the
identification of knowledge with true judgment. But Socrates contends
that one cannot make proper sense of the notion of "true judgment,"
unless one can explain what a false judgment is, a topic that also
emerges in such dialogues as Euthydemus, Cratylus, Sophist, Philebus,
and Timaeus. In order to examine the meaning of "false judgment," he
articulates five essentially abortive ways of looking at it: (a) false
judgment as "mistaking one thing for another" (188a–c); (b) false
judgment as "thinking what is not" (188c–189b); (c) false judgment as
"other-judgment" (189b–191a); (d) false judgment as the inappropriate
linkage of a perception to a memory – the mind as a wax tablet
(191a–196c); and (e) potential and actual knowledge – the mind as an
aviary (196d–200c).
The impossibility of false judgment as "mistaking one thing for
another" is demonstrated by the apparent plausibility of the following
perceptual claim: one cannot judge falsely that one person is another
person, whether one knows one of them, or both of them, or neither one
nor the other. The argument concerning false judgment as "thinking
what is not" rests on an analogy between sense-perception and
judgment: if one hears or feels something, there must be something
which one hears or feels. Likewise, if one judges something, there
must be something that one judges. Hence, one cannot judge "what is
not," for one's judgment would in that case have no object, one would
judge nothing, and so would make no judgment at all. This then cannot
be a proper account of false judgment. The interlocutors' failure
prompts a third attempt at solving the problem: perhaps, Socrates
suggests, false judgment occurs "when a man, in place of one of the
things that are, has substituted in his thought another of the things
that are and asserts that it is. In this way, he is always judging
something which is, but judges one thing in place of another; and
having missed the thing which was the object of his consideration, he
might fairly be called one who judges falsely" (189c). False judgment
then is not concerned with what-is-not, but with interchanging one of
the things-which-are with some other of the things-which-are, for
example beautiful with ugly, just with unjust, odd with even, and cow
with horse. The absurdity of this substitution is reinforced by
Socrates' definition of judgment as the final stage of the mind's
conversing with itself. How is it possible, then, for one to conclude
one's silent, internal dialogue with the preposterous equation of two
mutually exclusive attributes, and actually to say to oneself, "an odd
number is even," or "oddness is evenness"?
The next attempt at explaining false judgment invokes the mental acts
of remembering and forgetting and the ways in which they are
implicated in perceptual events. Imagine the mind as a wax block,
Socrates asks Theaetetus, on which we stamp what we perceive or
conceive. Whatever is impressed upon the wax we remember and know, so
long as the image remains in the wax; whatever is obliterated or
cannot be impressed, we forget and do not know (191d-e). False
judgment consists in matching the perception to the wrong imprint,
e.g., seeing at a distance two men, both of whom we know, we may, in
fitting the perceptions to the memory imprints, transpose them; or we
may match the sight of a man we know to the memory imprint of another
man we know, when we only perceive one of them. Theaetetus accepts
this model enthusiastically but Socrates dismisses it because it
leaves open the possibility of confusing unperceived concepts, such as
numbers. One may wrongly think that 7+5 = 11, and since 7+5 = 12, this
amounts to thinking that 12 is 11. Thus arithmetical errors call for
the positing of a more comprehensive theoretical account of false
judgment.
Socrates' next explanatory model, the aviary, is meant to address this
particular kind of error. What Aristotle later called a distinction
between potentiality and actuality becomes the conceptual foundation
of this model. Socrates invites us to think of the mind as an aviary
full of birds of all sorts. The owner possesses them, in the sense
that he has the ability to enter the aviary and catch them, but does
not have them, unless he literally has them in his hands. The birds
are pieces of knowledge, to hand them over to someone else is to
teach, to stock the aviary is to learn, to catch a particular bird is
to remember a thing once learned and thus potentially known. The
possibility of false judgment emerges when one enters the aviary in
order to catch, say, a pigeon but instead catches, say, a ring-dove.
To use an arithmetical example, one who has learned the numbers knows,
in the sense that he possesses the knowledge of, both 11 and 12. If,
when asked what is 7+5, one replies 11, one has hunted in one's memory
for 12 but has activated instead one's knowledge of 11. Although the
aviary's distinction between potential and actual knowledge improves
our understanding of the nature of episteme, it is soon rejected by
Socrates on the grounds that it explains false judgment as "the
interchange of pieces of knowledge" (199c). Even if one, following
Theaetetus' suggestion, were willing to place in the aviary not only
pieces of knowledge but also pieces of ignorance—thereby making false
judgment be the apprehension of a piece of ignorance—the question of
false judgment would not be answered satisfactorily; for in that case,
as Socrates says, the man who catches a piece of ignorance would still
believe that he has caught a piece of knowledge, and therefore would
behave as if he knew. To go back to the arithmetical example mentioned
earlier, Theaetetus suggests that the mistaking of 11 for 12 happens
because the man making the judgment mistakes a piece of ignorance for
a piece of knowledge but acts as if he has activated his capacity for
knowing. The problem is, as Socrates says, that we would need to posit
another aviary to explain how the judgment-maker mistakes a piece of
ignorance for a piece of knowledge.
Socrates attributes their failure to explain false judgment to their
attempting to do so before they have settled the question of the
nature of knowledge. Theaetetus repeats his definition of knowledge as
true judgment but Socrates rejects it by means of the following
argument: suppose, he says, the members of a jury are "justly
persuaded of some matter, which only an eye-witness could know and
which cannot otherwise be known; suppose they come to their decision
upon hearsay, forming a true judgment. Hence, they have decided the
case without knowledge, but, granted they did their job well, they
were correctly persuaded" (201b-c). This argument shows that forming a
true opinion about something by means of persuasion is different from
knowing it by an appeal to the only method by means of which it can be
known—in this case by seeing it—and thus knowledge and true judgment
cannot be the same. After the failure of this attempt, Socrates and
Theaetetus proceed to their last attempt to define knowledge.
d. Knowledge as True Judgment with Logos (201c – 210d)
Theaetetus remembers having heard that knowledge is true judgment
accompanied by Logos (account), adding that only that which has Logos
can be known. Since Theaetetus remembers no more, Socrates decides to
help by offering a relevant theory that he once heard.
According to the Dream Theory (201d-206b), the world is composed of
complexes and their elements. Complexes have Logos, while elements
have none, but can only be named. It is not even possible to say of an
element that "it is" or "it is not," because adding Being or non-Being
to it would be tantamount to making it a complex. Elements cannot be
accounted for or known, but are perceptible. Complexes, on the
contrary, can be known because one can have a true belief about them
and give an account of them, which is "essentially a complex of names"
(202b).
After Theaetetus concedes that this is the theory he has in mind, he
and Socrates proceed to examine it. In order to pinpoint the first
problematic feature of the theory, Socrates uses the example of
letters and syllables: the Logos of the syllable "so" – the first
syllable of Socrates' name – is "s and o"; but one cannot give a
similar Logos of the syllable's elements, namely of "s" and "o," since
they are mere noises. In that case, Socrates wonders, how can a
complex of unknowable elements be itself knowable? For if the complex
is simply the sum of its elements, then the knowledge of it is
predicated on knowledge of its elements, which is impossible; if, on
the other hand, the complex is a "single form" produced out of the
collocation of its elements, it will still be an indefinable simple.
The only reasonable thing to say then is that the elements are much
more clearly known than the complexes.
Now, turning to the fourth definition of knowledge as true judgment
accompanied by Logos, Socrates wishes to examine the meaning of the
term Logos, and comes up with three possible definitions. First,
giving an account of something is "making one's thought apparent
vocally by means of words and verbal expressions" (206c). The problem
with this definition is that Logos becomes "a thing that everyone is
able to do more or less readily," unless one is deaf or dumb, so that
anyone with a true opinion would have knowledge as well. Secondly, to
give an account of a thing is to enumerate all its elements (207a).
Hesiod said that a wagon contains a hundred timbers. If asked what a
wagon is, the average person will most probably say, "wheels, axle,
body, rails, yoke." But that would be ridiculous, Socrates says,
because it would be the same as giving the syllables of a name to
someone's asking for an account of it. The ability to do that does not
preclude the possibility that a person identifies now correctly and
now incorrectly the elements of the same syllable in different
contexts. Finally, giving an account is defined as "being able to tell
some mark by which the object you are asked about differs from all
other things" (208c). As an example, Socrates uses the definition of
the sun as the brightest of the heavenly bodies that circle the earth.
But here again, the definition of knowledge as true judgment with
Logos is not immune to criticism. For if someone, who is asked to tell
what distinguishes, say, Theaetetus, a man of whom he has a correct
judgment, from all other things, were to say that he is a man, and has
a nose, mouth, eyes, and so on, his account would not help to
distinguish Theaetetus from all other men. But if he had not already
in his mind the means of differentiating Theaetetus from everyone
else, he could not judge correctly who Theaetetus was and could not
recognize him the next time he saw him. So to add Logos in this sense
to true judgment is meaningless, because Logos is already part of true
judgment, and so cannot itself be a guarantee of knowledge. To say
that Logos is knowledge of the difference does not solve the problem,
since the definition of knowledge as "true judgment plus knowledge of
the difference" begs the question of what knowledge is.
The definition of knowledge as "true judgment plus Logos" cannot be
sustained on any of the three interpretations of the term Logos.
Theaetetus has nothing else to say, and the dialogue ends
inconclusively. Its achievement, according to Socrates, has been to
rid Theaetetus of several false beliefs so that "if ever in the future
[he] should attempt to conceive or should succeed in conceiving other
theories, they will be better ones as the result of this enquiry"
(210b–c).
Despite its failure to produce a viable definition of knowledge, the
Theaetetus has exerted considerable influence on modern philosophical
thought. Socrates' blurring of the distinction between sanity and
madness in his examination of knowledge as perception was picked up in
the first of Descartes' Meditations (1641); echoes of Protagorean
Relativism have appeared in important works of modern philosophy, such
as Quine's Ontological Relativity and Other Essays (1969) and Kuhn's
The Structure of Scientific Revolutions (1970); In Siris: A Chain of
Philosophical Reflexions and Inquiries Concerning the Virtues of
Tar-Water (1744), Bishop Berkeley thought that the dialogue
anticipated the central tenets of his own theory of knowledge; in
Studies in Humanism (1907), the English pragmatist F.C.S. Schiller saw
in the section 166a ff. the pragmatist account of truth, first
expounded and then condemned; and L. Wittgenstein, in Philosophical
Investigations (1953), found in the passage 201d–202b the seed of his
Logical Atomism, espoused also by Russell, and found it reminiscent of
certain theses of his Tractatus Logico-Philosophicus.
4. References and Further Reading
a. General Commentaries
* Bostock, D. Plato's Theaetetus. Oxford, 1988.
* Burnyeat, M. F. The Theaetetus of Plato. Trans. M.J. Levett.
Indianapolis and Cambridge, 1990.
* Campbell, L. The Theaetetus of Plato. 2nd Ed. Oxford, 1883.
* Cornford, F. M. Plato's Theory of Knowledge. The Theaetetus and
the Sophist of Plato. Trans. F. M. Cornford. London, 1935.
* McDowell, J. Plato: Theaetetus. Trans. J. McDowell. Oxford, 1973.
* Polansky, R. Philosophy and Knowledge: A Commentary on Plato's
Theaetetus. Lewisburg, 1992.
* Sedley, D. N. The Midwife of Platonism: Text and Subtext in
Plato's Theaetetus. Oxford, 2004.
b. Knowledge as Arts and Sciences
* Burnyeat, M. F. "The Philosophical Sense of Theaetetus'
Mathematics." Isis 69 (1978). 489–513.
* Burnyeat, M. F. "Socratic Midwifery, Platonic Inspiration."
Bulletin of the Institute of the Classical Studies 24 (1977). 7–16.
* Santas, G. "The Socratic Fallacy." Journal of the History of
Philosophy 10 (1972). 127–41.
c. Knowledge as Perception
* Bolton, R. "Plato's Distinction between Being and Becoming."
Review of Metaphysics 29 (1975/6). 66–95.
* Burnyeat, M. F. "Protagoras and Self Refutation in Plato's
Theaetetus." Philosophical Review 85 (1976). 172–95.
* Burnyeat, M. F. "Plato on the Grammar of Perceiving." Classical
Quarterly 26 (1976). 29–51.
* Burnyeat, M.F. "Idealism and Greek Philosophy: What Descartes
Saw and Berkeley Missed." Philosophical Review 90 (1982). 3–40.
* Cole, A. T. "The Apology of Protagoras." Yale Classical Studies
19 (1966). 101–18.
* Cooper, J. M. "Plato on Sense Perception and Knowledge:
Theaetetus 184 to 186." Phronesis 15 (1970). 123–46.
* Lee, E.N. "Hoist with His Own Petard: Ironic and Comic Elements
in Plato's Critique of Protagoras (Tht. 161–171)," in E.N. Lee and
A.P.D. Mourelatos (eds.) Exegesis and Argument: Studies in Greek
Philosophy Presented to Gregory. Vlastos. Assen, 1973. 225–61.
* Matthen, M. "Perception, Relativism, and Truth: Reflections on
Plato's Theaetetus 152 – 160." Dialogue 24 (1985). 33–58.
* McCabe, M.M. Plato and his Predecessors: The Dramatisation of
Reason. Cambridge, 2000.
* Modrak, D.K. "Perception and Judgment in the Theaetetus."
Phronesis 26 (1981). 35–54.
* Rowe, C.J. et al. "Knowledge, Perception, and Memory: Theaetetus
166B." Classical Quarterly 32 (1982). 304–6.
* Silverman, A. "Flux and Language in the Theaetetus." Oxford
Studies in Ancient Philosophy 18 (2000). 109–52.
* Waterlow, S. "Protagoras and Inconsistency." Archiv für
Geschichte der Philosophie 59 (1977). 19–36.
d. Knowledge as True Judgment
* Ackrill, J. "Plato on False Belief: Theaetetus 187–200." Monist
50 (1966). 383–402.
* Burnyeat, M.F. and J. Barnes, "Socrates and the Jury: Paradoxes
in Plato's Distinction Between Knowledge and True Belief."
Aristotelian Society Supplementary Volume 54 (1980). 173-91 and
193–206.
* Denyer, N. Language, Thought and Falsehood in Ancient Greek
Philosophy. London, 1991.
* Lewis, F.A. "Foul Play in Plato's Aviary: Theaetetus 195Bff," in
E.N. Lee and A.P.D. Mourelatos (eds.) Exegesis and Argument: Studies
in Greek Philosophy Presented to Gregory. Vlastos. Assen, 1973.
262–84.
* G.B. Matthews, G.B. "A Puzzle in Plato: Theaetetus 189b–190e,"
in David F. Austin (ed.) Philosophical Analysis: A Defense by Example.
Dordrecht, 1988. 3–15.
* Rudebusch, G. "Plato on Sense and Reference." Mind 104 (1985). 526–37.
* C.F.J. Williams, C.F.J. "Referential Opacity and False Belief in
the Theaetetus." Philosophical Quarterly 22 (1972). 289-302.
e. Knowledge as True Judgment with Logos
* Annas, J. "Knowledge and Language: The Theaetetus and the
Cratylus," in Malcolm Schofield and Martha C. Nussbaum (eds.) Language
and Logos: Studies in Ancient Greek Philosophy presented to G.E.L.
Owen. Cambridge, 1982. 95–114.
* Fine, G.J. "Knowledge and Logos in the Theaetetus."
Philosophical Review 88 (1979). 366–97.
* Gallop, D. "Plato and the Alphabet." Philosophical Review 72
(1963). 364–76.
* Morrow, G.R. "Plato and the Mathematicians: An Interpretation of
Socrates' Dream in the Theaetetus." Philosophical Review 79 (1970).
309–33.
* Ryle, G. "Letters and Syllables in Plato." Philosophical Review
69 (1960). 431–51.
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