In Philosophical Explanations, Robert Nozick suggested that knowing that some proposition, p, is true is a matter of being “sensitive” to p’s truth-value. It requires that one’s belief state concerning p vary appropriately with the truth-value of p as the latter shifts in relevant possible worlds. Nozick fleshed out this sketchy view with a specific analysis of what sensitivity entails. Famously, he drew upon this analysis in order to explain how common-sense knowledge claims, such as my claim to know I have hands, are true, even though we do not know that skeptical hypotheses are false. His explanation hinged on rejecting the principle that knowledge is closed under (known) entailment. In this chapter I will criticize Nozick’s view of knowledge as “sensitivity” to truth-value. In doing so, I mean to undermine his case against the closure principle and against the claim that we do not know that skeptical hypotheses are false.

I begin with a review of Nozick’s notion of sensitivity and his analysis thereof.