 Edward Craig (1990) has argued that we can better understand the concept of knowledge by examining the purpose or purposes it serves. In a similar way, perhaps, the concept of certainty can be rendered clearer by looking at the ways in which philosophers have tried to put certainty to use. Most fundamentally, certainties provide us with inputs to inference that are stable and secure. If they are immune to doubt, they can put an end to the demand for reasons that feature in the Pyrrhonist’s argument for general skepticism (see Sextus Empiricus, Outlines of Scepticism; Sosa 1997; Sienkiewicz 2019). If they have probability 1, they can simplify reasoning by allowing the reasoner to treat them as simply given, without having to attend to the risk of error for each individual premise or the accumulation of error as uncertain premises are combined. If they are indefeasible, they can allow us to reject any potential counterevidence as misleading.