On reflection, I think I may be wrong to say that he is begging the question, but not because of your counter-argument. If we start from the other end of the list, the first thing that is not unambiguously not a pumpkin is not necessarily the last unambiguous pumpkin you picked in the first pass. You have introduced the ambiguity that the author is trying to insist is not there.<p>My other claims stand, I think. The point is that his linear model is not an adequate fit to the problem (for example, the ambiguity you introduced really is there, but the model excludes it.) Maybe there is something like question-begging in thinking that this is an adequate model.