Whereas infallibilism supports (S2) by demanding that an agent should be able to know the denials of all error-possibilities, closure merely demands that the agent knows the denials of those error-possibilities that are known to be logical consequences of what one knows. For example, if one knows the ordinary proposition that one is currently seated, and one further knows that if one is seated then one is not a BIV, then one must also know that one is not a BIV. Conversely, if one does not know that one is not a BIV then, given that one knows the entailment in question (which ought to be uncontroversial), one thereby lacks knowledge of the ordinary proposition in question, just as (S2) says. And note that, unlike (S2), the plausibility of closure is not merely prima facie . After all, we reason in conformity with closure all the time in cases where we gain knowledge of previously unknown propositions via knowledge of other propositions and the relevant entailment. Indeed, closure is in this respect far more compelling than infallibilism, since what credibility the latter thesis has is gained by philosophical argument rather than by prima facie reflection on our actual epistemic practice. The theoretical burden imposed upon anyone who advocates the denial of (S2) is thus very strong, since it requires a principled rejection of the intuitive principle of closure.