The sophist is presented negatively, but he can be said to be someone who merely pretends to have knowledge or to be a purveyor of false knowledge only if right opinion and false opinion can be distinguished. It seems impossible to say that the sophist presents things that are not as though they were, or passes off "non-being" as "being," since this would suggest that non-being exists, or that non-existence exists. Otherwise, the sophist couldn't "do" anything with it. The Stranger suggests that it is Parmenides' doctrine of being and non-being that is at the root of this problem, and so proceeds to criticize Parmenides' ideas, namely that "it is impossible that things that are not are."