I first describe Dretske’s famous zebra example wherein one infers from “it’s a zebra” to “it’s not a mule disguised to look like a zebra”. The construction of such putative counterexamples to closure requires that we can have fallibly grounded knowledge. I describe what I call the “basis fallibilism” exploited by such examples and distinguish it from other kinds of fallibilism. After presenting a number of such counterexamples – which I call “Dretske cases” – I consider Jonathan Vogel’s critique of Dretske’s mobilization of them in arguing for closure. I argue that Vogel’s critique succeeds only if we capitulate to the skeptic. I next identify a number of intuitive reactions to Dretske cases, noting that, since those reactions are in tension with one another, no one view can claim to accommodate them all. I then present a generalization of the argument against closure that appeals to Dretske cases – the “argument by counterexample”. Closure advocates must reject one or more of the premises of that argument; different closure-preserving views can then be distinguished by which premise they deny. Chapters 3–8 review those distinct closure-preserving responses to that argument.