Socrates turns now to the second inspiration for saying that knowledge is perception. According to Heraclitus, he has already pointed out, everything in the sensible world is in constant flux. He and Theodorus remind us that the flux doctrine goes back at least to Homer, and perhaps much earlier, for according to the ancient myths things originate with Oceanus and Tethys, which are flowing streams (see ch. 21.2* above).[39]
On the other hand, Socrates points out, Parmenides and his follower Melissus claim that everything is one and unchanging. So those who want to recognize a world in which both motion and rest have a place are caught between a rock and a hard place.[40]
For now we must consider the Heraclitean doctrine. We need to recognize two kinds of motion: alteration (alloiōsis) and motion in place (phora). The former is a change of quality—here Plato has Socrates apologize for introducing a new word for that category, poiotēs—while the latter is a change of place. These two species of motion were first introduced in Plato’s Parmenides (without a term for ‘quality’).[41] If, then, the flux doctrine is true, everything is changing in every way, that is, both in quality and in place. Similarly, every action must be changing, which would imply that, for instance, seeing would be turning into hearing and vice versa. We will not, then, be in a position to say that anything is this way or that way, because of the constant flux. And man will not be the measure of things either, because nothing will be stable enough to be knowable.[42]
At this point Theaetetus breaks in and ask Socrates to explain the position of Parmenides’ Eleatic school, according to which everything is one and nothing changes. Socrates demurs, saying that, having in his youth met the elderly Parmenides, he was overawed by him and is doubtful that he can adequately explain his theory. In any case, they would lose the thread of the current investigation. Here Plato alludes to his dialogue Parmenides, which presumably he expects his readers to be familiar with, but fails to draw any firm conclusions from that inquiry.[43]
Turning back to the present discussion, Socrates makes a distinction between the organs through which we perceive things and the faculties by means of which we grasp them. In the former case, the objects of one organ are very different from those of another. For instance, we perceive colors through our eyes and sounds through our ears, but the organs in question cannot perceive the sense objects of the other organs. Yet we can determine that both kinds of sense objects exist and that they are unlike each other. How can we make these kinds of judgments concerning both kinds of sense objects? Not, surely, through the organs themselves, which are limited to their own special objects. The common qualities must be discerned by means of the soul itself, operating at a higher level than the sense organs, by comparing, contrasting, and evaluating.
“Knowledge, accordingly,” concludes Socrates, “consists not in sensory experiences, but in reasoning about them. The latter can reach being and truth, whereas the former cannot.” But if that is true, knowledge cannot be mere sense perception. [44] Theaetetus agrees.
[39] Plato Theaetetus 179e, 180d.
[40] Plato Theaetetus 180e-181a.
[41] Plato Theaetetus 181c-d, 182a-b; Parmenides 138b8-c1.
[42] Plato Theaetetus 182d-183c.
[43] Plato Theaetetus 183c-184a.
[44] Plato Theaetetus 184d-186e.