 Gorgias challenges this thesis in his treatise On Not Being (preserved in two slightly different versions in the pseudo-Aristotelian On Melissus, Xenophanes, and Gorgias and in Sextus Empiricus’ Adversus Mathematicos VII 65 ff.). It takes direct aim at Eleatic assumptions, and especially those of Parmenides, by arguing that (i) there isn’t anything; (ii) even if there is something, it cannot be known; and finally, (iii) even if something can be known, one could not inform anyone of this. The second part is of particular interest, since Gorgias offers a reductio ad absurdum of the Parmenidean thesis, especially when deployed in the positive way we have just seen in Protagoras. Given that we obviously can have in mind, for example, a chariot-team racing on the sea – in fact, just by reading this, you have put it in mind – it should follow from the Parmenidean thesis that there is a chariot-team racing on the sea; but (pace Protagoras) plainly there is not. If both these intuitions are maintained, the only option left is to reject the Parmenidean thesis itself: it is not the case that one can only think or speak of what is – thought is not so tightly bound to reality. Gorgias immediately goes on to argue for a sceptical conclusion, on the grounds that we have no basis for telling which mental states correspond to reality and which do not, and so cannot know anything at all. But this further argument rests on having shown that our ordinary intuitions about what we can think and what exists or obtains together undermine the Parmenidean thesis. (For further discussion, see Caston 2002a)