We demonstrate that counterfactuals prime a mental simulation mind-set in which relevant but potentially converse alternatives are considered and that this mind-set activation has behavioral consequences. This mind-set is closely related to the simulation heuristic (Kahneman & Tversky, 1982). Participants primed with a counterfactual were more likely to solve the Duncker candle problem (Experiment 1), suggesting that they noticed an alternative function for one of the objects, an awareness that is critical to solving the problem. Participants primed with a counterfactual were more likely to simultaneously affirm the consequent and select the potentially falsifying card, but without selecting the irrelevant card, in the Wason card selection task, suggesting that they were testing both the stated conditional and its reverse (Experiment 2). The increased affirmations of the consequent decreased correct solutions on the task — thus, the primed mind-set can bias or debias thought and action. Finally, Experiment 3 provides further evidence that counterfactual primes increase the accessibility of relevant alternatives. Counterfactual primes attenuated the confirmation bias in a trait hypothesis testing context by increasing the selection of questions designed to elicit hypothesis-disconfirming answers, but without increasing the selection of neutral questions. The nature of priming effects and the role of counterfactual thinking in biasing and debiasing thought and action are discussed.