Heidegger concludes his account of the Theaetetus by outlining what he calls Plato’s failure in interpreting the phenomenon of the pseudes doxa in terms of speaking rather than seeing. This is because dianoia, inner reflection, is understood as a speaking to ourselves about what is before us, and as such founded on logos (speech).
The mis-taking of pseudes doxa is therefore a failure of the intended predicate, i.e. what is said about the object before us. This leads Plato to grasp the essence of pseudos, un-truth, as the un-correctness of the proposition. Accordingly truth must also have its seat in the logos, rather than unhiddenness it becomes understood as the correctness of the proposition. This leads to our present understanding of truth as the “correspondence, grounded in correctness, between the proposition and the thing.” (2)
This goes hand in hand with the conception of doxa as proposition, which means its original dual character as look and view recedes. Rather than being there in the uncertain duality of seeming, the truth and un-truth of what is seen now “simply stand alongside each other, indeed they have opposing directions, they even exclude one another.” (227)