Walton will present a number of examples before beginning his analysis of petitio principii. It's reasonable to look critically at these examples as well as his analysis--do you think he is right about which ones exhibit the fallacy and which do not?

Walton's analysis turns on the idea of "evidential priority" and the conditions under which it is required. He doesn't say very much about what he means by "evidential priority," so you can gather a good deal from the way he uses the term. You have encountered a related idea in Hamblin's account of one form of the fallacy in Aristotle (Hamblin, pp. 75-77). It's worth remembering that this form of the fallacy, which seems to be roughly what Walton has in mind, is somewhat different from the dialectical fallacy that led to the term (on this, see Hamblin p. 74 and see HP p. 25, at the end of 167^a, for Aristotle's original account in the Sophistical Refutations).