For Kant, knowledge involves certainty. If “certainty” requires that the grounds for a given propositional attitude guarantee its truth, then this is an infallibilist view of epistemic justification. Such a view says you can’t have epistemic justification for an attitude unless the attitude is also true. Here I want to defend an alternative fallibilist interpretation. Even if a subject has grounds that would be sufficient for knowledge if the proposition were true, the proposition might not be true. And so there is sometimes still rational room for doubt. The goal of this paper is to present four different models of what “certainty” amounts to, for Kant, each of which is compatible with fallibilism.