Over the past decade, the question of whether there is a mental logic has become subject to considerable debate. There have been attacks by critics who believe that all reasoning uses mental models and return attacks on mental-models theory. This controversy has invaded various journals and has created issues between mental logic and the biases-and-heuristics approach to reasoning, and the content-dependent theorists. However, despite its pertinence to current issues in cognition, few cognitive scientists really know what the mental-logic theory is, and misapprehensions are prevalent. This volume is a comprehensive presentation of the theory of mental logic and its implications for cognition and development, including the acquisition of language. The theory offered here has three parts. Part I is the mental logic that contains a set of inference schemas. Part II is a reasoning program that applies the schemas in lines of reasoning, including a direct-reasoning routine and more sophisticated indirect-reasoning strategies. Part III of the theory is pragmatic, proposing that the basic meaning of each logic particle is in the inferences that are sanctioned by its inference schemas.