In this paper, I argue that presuppositional indefinites in English are positive polarity items, while non-presuppositional indefinites are not. This proposal solves the puzzle of why a presuppositional indefinite subject cannot take scope below negation in a matrix context, but can do so in a downward-entailing context (Spector 2004). I show that presuppositional indefinites can be identified when they're subjects of individual-level predicates or when a presuppositional interpretation is forced by Maximize Presupposition (Heim 1991). I demonstrate that presuppositional indefinites share the properties of other positive polarity items. Non-presuppositional indefinites, on the other hand, may freely take scope under negation.