Jowett: not-seeing

Soc. And seeing is knowing, and therefore not-seeing is not-knowing ? THEAETETUS  

Soc. None of that, he will reply ; I do not ask or bid you answer in what sense you know, but only whether you know that which you do not know. You have been proved to see that which you do not see ; and you have already admitted that seeing is knowing, and that not-seeing is not-knowing : I leave you to draw the inference. THEAETETUS

Soc. Then we must not speak of seeing any more than of not-seeing, nor of any other perception more than of any non-perception, if all things partake of every kind of motion ? THEAETETUS