The Donald Broadbent Memorial Lecture

How we can improve reasoning

Philip Johnson-Laird

Princeton University, USA

Donald Broadbent believed that psychology should have practical applications, and that their investigation was crucial. This talk follows his advice. Human beings differ vastly in their ability to reason. The talk presents a theory that reasoning depends on envisaging possibilities. This theory is based on three main principles: individuals represent possibilities in mental models, and each mental model represents a different possibility; models are 'iconic', i.e., their structure corresponds to the structure of what they represent; and models normally represent only what is true. A conclusion is judged to be valid if it holds in all such mental models of the given information, and probable if it holds in most of them. The talk outlines the evidence corroborating the theory. It then discusses procedures for improving reasoning. Two such procedures, investigated by Victoria Bell, Malcolm Bauer, and others, make explicit for individuals the notion of keeping track of possibilities. They speed up reasoning and enhance its accuracy. Humans can learn to improve their reasoning.