A remarkable feature of human intelligence is our ability to think hypothetically. We can imagine how things might be in the future and how they might have turned out differently in the past. Imagination plays an important role in science, arts, and engineering, and is fundamental to rational decision making. A hypothesis is a supposition concerning some aspect of the world about us. Developing and testing hypotheses in an informal way is part of our everyday thinking. ‘Thinking hypothetically’ discusses hypothesis testing and scientific reasoning, introducing the concept of the mental model. It also discusses the Wason selection task—probably the most famous problem in the psychology of reasoning—as well as causal and counterfactual reasoning.