To be rational is to be able to reason. Thirty years ago psychologists believed that human reasoning depended on formal rules of inference akin to those of a logical calculus. This hypothesis ran into difficulties, which led to an alternative view: reasoning depends on envisaging the possibilities consistent with the starting point-a perception of the world, a set of assertions, a memory, or some mixture of them. We construct mental models of each distinct possibility and derive a conclusion from them. The theory predicts systematic errors in our reasoning, and the evidence corroborates this prediction. Yet, our ability to use counterexamples to refute invalid inferences provides a foundation for rationality. On this account, reasoning is a simulation of the world fleshed out with our knowledge, not a formal rearrangement of the logical skeletons of sentences.H uman beings are rational-a view often attributed to Aristotle-and a major component of rationality is the ability to reason (1). A task for cognitive scientists is accordingly to analyze what inferences are rational, how mental processes make these inferences, and how these processes are implemented in the brain. Scientific attempts to answer these questions go back to the origins of experimental psychology in the 19th century. Since then, cognitive scientists have established three robust facts about human reasoning. First, individuals with no training in logic are able to make logical deductions, and they can do so about materials remote from daily life. Indeed, many people enjoy pure deduction, as shown by the world-wide popularity of Sudoku problems (2). Second, large differences in the ability to reason occur from one individual to another, and they correlate with measures of academic achievement, serving as proxies for measures of intelligence (3). Third, almost all sorts of reasoning, from 2D spatial inferences (4) to reasoning based on sentential connectives, such as if and or, are computationally intractable (5). As the number of distinct elementary propositions in inferences increases, reasoning soon demands a processing capacity exceeding any finite computational device, no matter how large, including the human brain.