Practical Certainty

When we engage in practical deliberation, we sometimes engage in careful probabilistic reasoning. At other times, we simply make flat out assumptions about how the world is or will be. A question thus arises: when, if ever, is it rationally permissible to engage in the latter, less sophisticated kind of practical deliberation? Recently, a number of authors have argued that the answer concerns whether one knows that p. Others have argued that the answer concerns whether one is justified in believing that one knows that p. Against both of these, this paper argues that the answer concerns whether p is ‘practically certain’—that is, whether the actual epistemic probability that p differs from epistemic certainty that p only in ways that are irrelevant to the decision one currently faces.