Individuals, who know no logic, are able to make deductive inferences. For many years, psychologists argued that deduction depends on an unconscious system of formal rules of inference akin to those in proof-theoretic logic. The first mental model theory is for simple inferences based on quantifiers, and programs have simulated various versions of this theory, and the probabilistic theory often makes unsatisfactory predictions. The theory of mental models posits that the engine of human reasoning relies on content. The simulation of model theory concerns sentential reasoning, and it shows how an apparently unexceptional assumption leads to a striking prediction of systematic fallacies in reasoning - a case that yields crucial predictions about the nature of human deductive reasoning. The chapter concludes with an attempt to weigh up the nature of human rationality in the light of other simulation programs.