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The guiding inquiry of Plato’s Theaetetus is the nature of knowledge, but the dialogue offers no apparent answers. It demonstrates what knowledge is not, but never explicitly reveals what Plato holds knowledge to be. Something of his theory of knowledge is, however, smuggled in, suggested implicitly by the premises guiding the dialectic.
The first sign of this occurs almost immediately. Theaetetus says, in response to Socrates:
I think the things one can learn from Theodorus are knowledge—geometry and all the sciences you mentioned just now, and then there are the crafts of the cobbler and other workmen. Each and all of these are knowledge and nothing else.1
Socrates responds by mocking the young student. “When you are asked for one simple thing, you offer a whole variety.” Socrates wants to know about “the thing itself,” not about its various sorts. He calls it absurd to respond to a question about the nature of a thing with specific instances of that thing. Socrates wants a “short and simple” answer. The nature of “clay is earth mixed with moisture, never mind whose clay it may be.” What, then, is the nature of knowledge?
To this, Theaetetus responds: “It seems to me that one who knows something is perceiving the thing he knows, and, so far as I can see at present, knowledge is nothing but perception.” (emphasis added)2 Where did this response come from? At Socrates’ urging, Theaetetus blurts out the first thing that pops into his head. He does not give any evidence for his response; it seems true to him, and that is enough to satisfy Socrates.
Implicit in this exchange, in its switch from a list of particulars to a groundless unity, is the separation of the one and the many, the universal and the particular. Socrates does not recognize a list of the sorts of knowledge as a first step in discovering the nature of knowledge; but a unity, so long as it seems true, is worthy of praise. “Good. That is the right spirit in which to express one’s opinion,” says Socrates to Theaetetus’ second response. Already, then, Plato’s theory of knowledge comes through. The particular is always bad, and the universal always good, regardless of their truth or falsity. Induction, this tells us, is not a means to discovery. One must ignore the particulars and seek direct knowledge of the universal.
Socrates suggests that there is a connection between the one and the many: “[T]ry to find a single formula that applies to the many kinds of knowledge.”3 But this does not contradict the idea that knowledge comes from ignoring the many and embracing the one. For, if Plato’s theory of forms be taken into account, it suggests the one creates the many (this world is a reflection of the world of forms), but not that the one is understood through the many (the forms must be grasped directly). Instead, it suggests the opposite: that the many must be understood through the one. Only when one understands the universal forms will one understand the particular reflections of those forms.
Socrates goes on to offer an argument against the claim that perception is knowledge. This argument is another hint at Plato’s belief that not only is knowledge not the same as perception, but that it is antithetical to it. He has already divorced the universal and the particular by rejecting Theaetetus’ first response. Knowledge, then, must be found in one or the other; it cannot be found in both, since they are separate. So in arguing against perception, he is arguing against the particular. When perception is refuted, the particular is refuted as well, and only the universal remains as a possible realm of knowledge.
Nevertheless, one might have argued that one can reason from the particular to the universal by means of an account (logos), and that is why the second main argument of the dialogue is necessary. By eliminating the ideas that perception and perception-based belief with an account are knowledge, only one idea is left: knowledge is something that transcends perception entirely.
A more detailed examination of this argument reveals even more about Plato’s theory of knowledge. Worthy of notice is the use of doxa to represent perception-based belief or opinion. This word suggests not just that something is opinion, but that it is “mere opinion” or “conjecture”.4 With the modifying adjective alethes this creates the sense that what is being discussed is not a true grasp of reality, but a fanciful opinion that just happens to correspond with the way things are.
Socrates and Theaetetus discuss three types of accounts: speech, division into elements, and differentiation. The first is swiftly and rightly rejected as a means to knowledge. “If a man is not born deaf or dumb he can signify what he thinks on any subject.”5 The second is rejected on the grounds that mere knowledge of a thing’s parts does not give knowledge of the whole. For one can know the individual letters of a syllable, but still be ignorant of the syllable as a whole. The third, that an account is “being able to name some mark by which the thing one is asked about differs from everything else”6, is also rejected. For if an account is knowledge of differentness, then knowledge would be, “Correct belief together with knowledge of a differentness.” But it is, says Socrates, silly to claim that knowledge is the addition of knowledge to belief. That formulation does not answer the question: what is knowledge?
After rejecting these three types of accounts as means to knowledge, Socrates and Theaetetus conclude that “neither perception, nor true belief, nor the addition of an ‘account’ to true belief can be knowledge.” This leaves only one option: that knowledge is not to be found in perception at all, but in something that transcends perception. This corresponds to other dialogues, in which the real being in which one must look for knowledge is the world of forms.
Further, there is one type of account left unmentioned, and this omission reveals once and for all, in its entirety, the theory of knowledge implicit throughout the dialogue. This missing account is a causal, inductive one. One observes particular instances, finds a common cause, and generalizes. To obtain knowledge that man is mortal, for example, one would observe particular men, find the common cause (suppose it is the aging process), and generalize that all men are mortal. Plato might argue as he does with differentiation that this does not tell us what knowledge is, since it already requires knowledge of the aging process. But a hierarchical theory of knowledge, in which generalizations rest on previous generalizations, and those on particulars, and those particulars on certain axioms, takes care of that objection. The mere fact that knowledge is partially derivative from previous knowledge does not invalidate it.
Why would Plato omit this possibility? Part of the reason may stem from his view of causality. In his Phaedo, he explains causality as follows:
Somehow it seemed right that the mind should be the cause of everything, and I reflected
that if this is so, mind in producing order sets everything in order and arranges each
individual thing in the way that is best for it. Therefore if anyone wished to discover the reason why any given thing came or ceased or continued to be, he must find out how it was best for that thing to be, or to act or be acted upon in any other way.7
Causality, then, is based on logical necessity apart from perception. If the earth is round, the cause of its being round is that it is better that way. Mind, not matter, is the cause of things. This is why a causal account based on perception would not occur to Plato.
That Plato omits and thereby denies the possibility of derivative knowledge implies that it is impossible to know one thing before another thing. Knowledge of one thing, then, could only come with knowledge of everything. One is either completely ignorant or omniscient. This is precisely what is argued for in The Republic:
[T]he objects of knowledge not only receive from the presence of the good their being known, but their very existence and essence is derived to them from it, though the good itself is not essence but still transcends essence in dignity and surpassing power.8
If something is to be known, then, one must first know the good. And if one knows the good, one knows everything, since everything is derived from it. There are, then, only two possibilities: ignorance and omniscience. Derivative knowledge, however, would imply that some knowledge is possible before one has all knowledge. This is why the possibility of knowledge based on previous knowledge would not occur to Plato.
Theaetetus, when he gives three responses to Socrates’ question which all rely in some way on this-worldly perception, expresses what Plato would characterize as a cave mentality. By rejecting all three answers, Plato urges the reader to step out of the cave and into the light of true being.
An examination of Book VII of The Republic will make this clearer. The analogy of the cave compares our knowledge of the world to men in a cave who all their lives see only shadows on the wall. Though they see only shadows, they suppose their visions to be real things. But one of these men might be “freed from his fetters and compelled to stand up suddenly and to lift up his eyes to the light.” He would then realize that all he had seen before had been an illusion. “And if […] someone should drag him thence by force up the ascent which is rough and steep […]”9, he would then be able to see reflections in water, and then the things themselves, and then the stars by night, and then the sun’s light at day. And the sun is the source of all being, the source of all knowledge. So, too, are real men born into a world of illusion: the world of becoming. To free themselves of it, they must ignore deceptive appearances and contemplate pure essence. Plato then describes the rigorous and abstract studies necessary to draw the soul from the visible to the intelligible. Only when it leaves this world entirely and contemplates the form of the good, which corresponds to the sun in the cave analogy, will one have knowledge.
One sees in this the very same ideas presented implicitly in Theaetetus: the necessity to understand the many through the one, the insignificance of perception, the omission of causality, and the rejection of derivative knowledge. Despite the seeming lack of conclusions in the dialogue, then, it turns out that Plato’s theory abounds throughout the dialogue through the rejection of its apparent antitheses, various forms of perception-based knowledge, and through the neglecting of its real antithesis, induction of the universal from the particular.
Endnotes
1. All Theaetetus quotes from Plato’s Theory of Knowledge: the Theaetetus and the Sophist, translated by Francis Macdonald Cornford. 146d.
2. Theaetetus, 151e.
3. Theaetetus, 148d.
4. “dñja."An Intermediate Greek-English Lexicon. Liddell and Scott’s, 7th ed.
5. Theaetetus, 206d.
6. Theaetetus, 208c.
7. Phaedo from The Last Days of Socrates, translated by Hugh Tredennick. 97c.
8. Plato: The Republic, with an English translation by Paul Shorey. 509b. (Loeb Classical Library, Cambridge, Mass., and London, 1953, 1956; first printed, 1930)
9. Plato: The Republic, 515e.
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