THE ESSENCE OF TRUTH
Because Theaetetus is not yet free to learn to see, because he is still too much under the influence of commonly accepted principles and concepts into which he forces the phenomenon and thus deforms it. More precisely, Socrates leads him into this error by emphasizing the vulgar meaning of ψευδής (as 'null') and allowing him to fall into the trap. But in this Theaetetus is only the representative, as it were, of healthy common sense - to be sure, the healthy common sense which speaks and thinks in Greek.
There are three factors here which determine the common understanding and make its view unfree: at bottom, the same as what already hindered the first attempt.
1. Just as there δοξάζειν was οἴεσθαι (to take something as something), so now, compelled by the phenomenon, it must again be conceived as δοξάζειν τι περί τινος, believing something about something, having a view about something, seeing something as this and as something else. It is therefore not to be overlooked that the object of δόξα is as it were i objects (ἀμφότερα). To the phenomenon 'having a view of something' there belongs the 'whereof and the view of it. But we call 'having a view' δοξάζειν, and this is always δοξάζειν τι, intending something. So the obvious thing is to equate δοξάζειν with such modes of comportment as ὁρᾶν τι, ἀκούειν τι, therefore with αἴσθησις; what holds for this (seeing and hearing) must also hold for δοξάζειν (opining). Theaetetus falls into this trap. At the moment when he embraces this correspondence and understands δοξάζειν in the manner of seeing, hearing and the like, he fails to pay due regard to the object of belief, taking it likewise as a simple object. The object is given up and sinks out of sight; i.e. the ἀμφότερα, the double-aspect of the object of δόξα, is not held on to, and so what is further said is groundless.
2. If ψευδὴς is the null, ψευδὴς δόξα null opinion, and the null the same as the non-existent, μὴ ὄν, then the distorted view cannot have any object at all. What is not something is nothing (οὐδέν); the non-existent, which takes the lead here, is equated with the nothing. It is not at all asked whether the non-existent is also a being and can be such. For how otherwise could a distorted view be resisted, if from the beginning it were nothing! Here also the phenomenon comes into view in a certain way, in that someone who has a distorted view believes something about the object (something which does not apply, thus a nothing); therefore the impossibility of the phenomenon.
3. Finally, and in connection with this, the equating of the non-existent and the nothing is strengthened by the fact that beings exclude the nonexistent, exactly as in the previous case knowing excluded not-knowing,