 The sophists Protagoras and Gorgias are of particular interest. According to Plato, Protagoras held that one cannot believe what is not and that whatever one experiences is true (Theaetetus 167a). This fits snugly with Protagoras’ widely attested doctrine that each human being is “the measure of all things,” which, on any interpretation, excludes the possibility of error. But Protagoras’ endorsement reveals an interesting subtlety in the Parmenidean thesis that what is not cannot be thought about – or more positively, the thesis that if something is had in mind, it must be. The goddess in Parmenides’ poem uses this thesis in a negative fashion, moving from the assumption that certain things ‘are not’ to the paradoxical conclusion that we cannot actually think about them either. But one might equally use it in a positive fashion, as Protagoras seems to have done, to move from the assumption that we can in fact think of such things to the conclusion that they must in fact ‘be’, i.e. exist or obtain, after all. In itself, the Parmenidean thesis only makes a conditional claim, and prima facie it seems open to employ either modus tollens or modus ponens. Depending on whether one retains commonplace assumptions about what we can think or about what there is in the world, the Parmenidean thesis can lead to either a bloated ontology or a very restricted psychology.