Oaksford and Chater (O&C) have written a book of great interest, which has advanced the new paradigm in the psychology of reasoning. As their title implies, they are Bayesians and see an essential connection between reasoning and probability judgement. They attack the claim that rationality is about following the rules of logic, and argue for Bayesian probability theory as the correct normative and computation theory of human reasoning. Logic is correctly used in pure mathematics, but ordinary human beings cannot escape uncertainty. Why would people simply assume that they win a fortune if they buy a lottery ticket? By making this assumption, and then buying a ticket, they can validly infer that they will win the fortune. But that inference is useless for a rational belief and decision making about the lottery and buying a ticket. From this perspective, O&C propose new theories of conditional reasoning, Wason’s selection task (both indicative and deontic), and syllogistic inference. O&C’s book has already been extensively reviewed in Behavioral and Brain Sciences, and they have extended andmodified their proposals in replies to the commentaries (Oaksford & Chater, 2009a, 2009b). I will not repeat here reviewing points that have been covered adequately in those commentaries. But I believe that fundamental aspects of O&C’s approach have not yet had sufficient attention. These are the relation between Bayesian rationality and logic, and the analysis of conditional reasoning in termsof the centralBayesian concept of conditional probability. The bestway to introduce these topics, and explain O&C’s contribution to the new paradigm, is by describing the old ‘‘logicist’’ paradigm, as O&C call it. I will argue that their use of ‘‘logicist’’ is misleading and prefer ‘‘binary paradigm’’ for the old one. The best example of a book in the binary paradigm is Johnson-Laird and Byrne (1991). In their mental model theory, every proposition is only THINKING & REASONING, 2009, 15 (4), 431–438
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