The dialogue that constitutes the sequel to the Parmenides is the Theaetetus. It begins with a short introduction in which Euclides of Megara, the philosopher who had hosted the Socratics in a retreat after the death of Socrates (see above, ch. 6.1*), shares with his friend and countryman Terpsion a manuscript in which he recounts a conversation between Socrates and Theaetetus.[23] The young man was a promising student of the mathematician Theodorus and came to be a member of Plato’s Academy.[24] At the time of the reading, Theaetetus has recently died, of wounds received in battle and a subsequent disease, so that the dialogue represents a memorial to him. Theaetetus died around 369 BCE, so Plato’s dialogue has a terminus post quem.[25] Euclides mentions that his manuscript is written like the script of a play, with names of the speakers attached to each speech, to avoid tedious narrative interjections of who said what.[26] It seems likely, as is often remarked, that Plato grew tired of the endless ‘he said’s in the Parmenides. He had occasionally used the script format in dialogues before; now Plato will use it exclusively in his dialogues forever after.
In the dialogue, Theodorus introduces his student Theaetetus to Socrates, praising his sharp mind and good character. Socrates engages him in a search for a definition of knowledge, very much in the style of early Socratic dialogues of definition—except that the object to be defined here is not a virtue. After Theaetetus offers a list of examples rather than a definiens, Socrates points out that the definition should be general and not particular. Theaetetus is able to connect the request with definitions of mathematical items. Socrates goes on to reveal that his mother was a midwife and that he has inherited the ability to marry ideas from his interlocutors with ideas from famous thinkers to see if they will produce good offspring.
The skill midwifery has often been attributed to the historical Socrates; but in fact, from what we gather in the early or Socratic dialogues of Plato, Socrates’ proper method precludes intellectual midwifery.[27] Socrates insists on the Say What You Believe principle; even if you get a definition or belief from a famous thinker, when you engage in a Socratic inquiry, you must advance propositions that you personally accept.[28] If you engage with Socrates, he will be examining what you think, not what some sophist or wise person says. That way you will be refuted or vindicated, not some famous character who is not present. Now, in the Theaetetus, Plato seems to welcome speculative explorations of philosophical concepts. His aim is not to perform some kind of philosophical therapy on the individual, but to discover, in company with the answerer, a satisfactory account of the conception in question, in this case: knowledge.
With that preamble in place, Theaetetus offers a promising definition: “knowledge is nothing other than perception.”[29] Theaetetus has in mind specifically sense perception. Socrates promises to test his definition to see if it will be a viable offspring. In accordance with his profession of intellectual midwifery, Socrates calls up the famous utterance of Protagoras the sophist: “Man is the measure of all things, of things that are that they are; of things that are not that they are not.”[30] If that is right, then each individual is the judge of what things are or are not for so him or her, through the individual’s own sense perceptions.
But this brings up another philosophical association. Since we encounter the world through sense perception, and perception is a kind of interaction between us and the world, and both we and the world are constantly changing, there must be a universal change and flux, as asserted by Heraclitus and Homer, with Oceanus and Tethys. “So from everything we have said from the beginning, nothing is one thing itself by itself, but being must be totally annihilated … and we ought not [to use this term] or apply any other name, but express things according to nature as ‘coming-to-be,’ ‘emerging,’ ‘perishing,’ ‘altering.’”[31] In a proper language, verb forms will replace nouns in describing our ever-shifting experience.
Our experience is, on this theory, ineluctably our own. “Necessarily, whenever I become perceptive, I become perceptive of something, for it is impossible to become perceptive of nothing. And that thing becomes for someone when it becomes sweet or bitter or something like that … It turns out, then, I suppose, that we are, if we are, or become, if we become, for each other.” The perceiver and the perceived are tethered to each other. “So if someone says ‘someone is something’ or ‘becomes something,’ one must add ‘for someone’ or ‘of something’ or ‘in relation to something.’ But one must not say anything is or comes-to-be something by itself …”[32]
Summing up, Socrates says, “In conclusion Homer, Heraclitus, and their whole tribe concur in asserting that everything is in motion like a stream, and according to Protagoras the Wise, man is the measure of all things. And according to Theaetetus, in light of these facts, knowledge proves to be perception.”[33]
[23] Plato Phaedo 59c tells us that both were present on Socrates’ last day.
[24] Diogenes Laertius 3.6 has Plato visiting Theodorus of Cyrene at his city, a Greek colony on the coast of modern-day Libya; but this story is dubious: see ch. 6.1 above.
[25] It is usually assumed the dialogue was written within a year or two Theaetetus’ death. For reasons that will become apparent, I would date it more than two years later.
[26] Plato Theaetetus 143b-c.
[27] Burnyeat 1977.
[28] Vlastos 1983.*
[29] Plato Theaetetus 151e.
[30] Plato Theaetetus 152a = Protagoras fr. 1.
[31] Plato Theaetetus 157a-b.
[32] Plato Theaetetus 160a-c.
[33] Plato Theaetetus 160d-e.