Van Inwagen's Modal Skepticism and The Scope of Modal knowledge

There is a famous idea in modal epistemology according to which conceivability of a proposition is a good guide for its possibility. Yablo (1993) persents a model for justification of modal beliefs, based on which Conceivability of a proposition is evidence for its possibility. Van Inwagen (1998) believes that if we accept Yablo’s model, we have to accept modal skepticism. To argue for this, Van Inwagen examines the modal status of the proposition that transparent iron exists on Yablo’s model. Van Inwagen claims that this proposition is undecidable on Yablo’s model. So we cannot have a justified belief that it is possible that transparent iron exists. If van Inwagen’s claim about the modal status of the proposition that transparent iron exists is correct, Yablo’s model, one might think, faces a serious problem. For if we generalize van Inwagen’s analysis of the proposition that transparent iron exists, we have to count intuitively possible propositions, propositions the modal status of which can be intuitively known as possible, as undecidable. But it is quite plausible that our beliefs about the possibility of some intuitively possible propositions are justified, so these propositions are not undecidable. I will, however, argue that van Inwagen’s analysis of modal status of the proposition that transparent iron exists cannot be generalized to all (or most) intuitively possible propositions. And therefore it is possible to accept at the same time both Yablo’s justification model and van Inwagen’s analysis about the modal status of propositions like transparent iron exist.