Several years ago Mr. J. Xenakis proposed an interpretation of some aspects of the passage in the Theaetetus in which the thesis that sLa 'tFc is cx'L0atOLq receives its final refutation (184B4186E12).1 Although I agree in the main with his interpretation, I believe that it can be supported more strongly. Thus an analysis of this passage is the first task of this paper. But on the basis of that analysis, I want to show also how this passage is related to what follows in the dialogue, the long discussion beginning with the thesis that knowledge is true opinion but turning almost immediately to a discussion of the nature and possibility of false opinion and ending with the wax block and aviary models (187A1-200D4).2 The criticism of the thesis that enL r'pn is aOt-aLa begins with the distinction between what the soul apprehends through the senseorgans and what it apprehends by itself. Whatever Plato's views about the soul might be at his writing of the Theaetetus, he wants to distinguish between those characteristics which the soul apprehends through the sense-organs and those characteristics which are called common to everything (&rl 7raaL xoLvov -184C4-5). In spite of his expressed desire to be precise (184 C 1-7), Plato has some difficulties and ambiguities in his terminology. His general point is to show that knowledge cannot be equated with aX'LaOcGL4. But he does speak, on the one hand, of what the soul perceives (odaO&voXuCXL -184E8-185A2) when he talks about the particular objects of the various senses, e.g. colors, tastes, sounds, etc. On the other hand he also speaks of what the soul perceives (again xLaOCXVO[LCXL -185 C8) when he talks about the XOLVa, characteristics which are somehow different from those apprehended via the individual sense-organs. The ambiguity in aLsaO%vo[LaL leaves two questions to be answered.