Taking a philosophical approach based on what Plato, Aristotle, and Descartes said about knowledge, this paper addresses some of the murkiness in the conceptual space surrounding the issue of whether prior knowledge does or does not facilitate text comprehension. Specifically, the paper first develops a non-exhaustive typology of cases in which prior knowledge might not facilitate text comprehension. The paper then examines whether any of a subset of the cases, those in which prior knowledge hurts text comprehension, are really possibilities. The subset examined consists of 12 case-types in which the use of propositional knowledge in the comprehension process hurts propositional outcomes. (MS)