John MacFarlane

We consider a paradox involving indicative conditionals (‘ifs’) and deontic modals (‘oughts’). After considering and rejecting several standard options for resolv- ing the paradox—including rejecting various premises, positing an ambiguity or hidden contextual sensitivity, and positing a non-obvious logical form—we offer a semantics for deontic modals and indicative conditionals that resolves the paradox by making modus ponens invalid. We argue that this is a result to be welcomed on independent grounds, and we show that rejecting the general validity of modus ponens is compatible with vindicating most ordinary uses of modus ponens in reasoning.

Recent years have seen an explosion of interest in the semantics of knowledge-attributing sentences, not just among epistemologists but among philosophers of language seeking a general understanding of linguistic context sensitivity. Despite all this critical attention, however, we are as far from consensus as ever. If we have learned anything, it is that each of the standard views—invariantism, contextualism, and sensitive invariantism—has its Achilles’ heel: a residuum of facts about our use of knowledge attributions that it can explain only with special pleading. This is not surprising if, as I will argue, there is a grain of truth in each of these views.

According to “sensitive invariantism,” the word “know” expresses the same relation in every context of use, but what it takes to stand in this relation to a proposition can vary with the subject’s circumstances. Sensitive invariantism looks like an attractive reconciliation of invariantism and contextualism. However, it is incompatible with a widely-held view about the way knowledge is transmitted through testimony. If both views were true, someone whose evidence for p fell short of what was required for knowledge in her circumstances could come to know that p simply by feeding her evidence to someone in less demanding circumstances and then accepting his testimony.