This point leads us to some further consideration of the nature of the “evident” starting-points. Allowing that Galen is justified in his reliance on empirical evidence, things evident to the senses (see further below on his anti-Skeptical arguments), we still need to analyse further what propositions may count as evident to reason. Relevant here is his own classification, already mentioned, of different grades of argument on the basis of different kinds of premiss. In language indebted to Aristotle (in particular to his Sophistical Refutations and Topics) and the Aristotelian tradition, though in its precise formulation apparently unique to Galen, he lists four: epistemonic, dialectical, rhetorical and sophistic (PHP II.3, 110–12; II.8, 156–8). This last type, which relies on ambiguity or linguistic confusion, need not detain us further. The first type corresponds to the category which we have already considered, that of “proceeding directly from the ousia itself of the subject under enquiry”. Dialectical premisses are ones which do not proceed thus directly from the ousia of the subject, but which may nonetheless be useful for the training of the young, for refuting sophists, for leading to discoveries or for producing aporia. Rhetorical premisses derive from generally accepted (endoxa) or everyday examples or inductions, or from the evidence of witnesses (which may include laypersons, poets or philosophers).