Deduction

How do people make deductions? The orthodox answer is that they follow formal rules of inference. Originally published in 1991, and reissued here with a new preface, the authors of Deduction repudiate this theory. They argue that people reason by building a model of the state of affairs, formulating a conclusion based on this model, and searching for alternative models that refute it. Formal rules work syntactically; mental models work semantically. The theories therefore make different predictions about the difficulty of deductions. The book reports experiments that compared these predictions in the main domains of deduction: propositional reasoning; relational reasoning; and quantificational reasoning. In each domain, the results corroborated the model theory and ran counter to the rule theories.

The authors relate their findings to problems in artificial intelligence, linguistics and anthropology. They describe computer programs based on the model theory, including one that solves a major problem in the design of electronic circuits. Finally, they show how the theory resolves a long-standing controversy about rationality.

Phil Johnson-Laird is emeritus Stuart professor of psychology at Princeton University after earlier academic positions in England. Ruth Byrne and his collaboration began in 1986 at the MRC’s psychological unit in Cambridge and it led to Deduction. Phil continues to do research in collaborations with Ruth and other friends into reasoning, creativity, emotions, and psychological illnesses. He has published over a dozen books and several hundred papers. This research has been recognized in elections to learned societies in Britain (FRS, FBA) and America (NAS, APS), in honorary degrees, and in the award of the Fyssen prize for studies in rationality.

Ruth Byrne is the Professor of Cognitive Science at Trinity College Dublin. Her research on reasoning and imaginative thought has been published in over 150 papers and her former books include The Rational Imagination (2005). She is a member of the Royal Irish Academy and a Fellow of the Cognitive Science Society. She has been awarded the Royal Irish Academy’s Gold Medal, and the European Society for Cognitive Psychology’s Broadbent Prize.