Legal scholarship, following rational choice theory, has traditionally treated uncertainty as a single category. A large body of experimental studies, however, has established that individuals treat guesses concerning the future differently than guesses concerning the past. Even where objective probabilities and payoffs are identical, individuals are much more willing to predict a future event (and are more confident in the accuracy of their predictions) than they are willing to postdict a past event (and are also less confident in the accuracy of their postdiction). For example, individuals are more willing to bet on the results of a future die toss than they are willing to bet on the results of a past toss. After presenting the robust psychological and experimental economic literature, this Article demonstrates the relevance of the behavioral differences concerning past and future uncertainties for legal policy. It shows that the prediction-postdiction findings are important for the design of legal norms, for the choice among competing law-enforcement strategies and for the application of various sentencing practices. The making of legal norms, the detection of violators and the infliction of sanctions, it is shown, may generate different types of uncertainty involving predictions and postdictions, and those can be exploited by policymakers in order to provide optimal incentives.