Jonathan St B T Evans

The probability of conditionals: The psychological evidence

The two main psychological theories of the ordinary conditional were designed to account for inferences made from assumptions, but few premises in everyday life can be simply assumed true. Useful premises usually have a probability that is less than certainty. But what is the probability of the ordinary conditional and how is it determined? We argue that people use a two stage Ramsey test that we specify to make probability judgements about indicative conditionals in natural language, and we describe experiments that support this conclusion. Our account can explain why most people give the conditional probability as the probability of the conditional, but also why some give the conjunctive probability. We discuss how our psychological work is related to the analysis of ordinary indicative conditionals in philosophical logic.

The Nature of Reasoning: The Role of Prior Belief in Reasoning

In this chapter we examine research that uses well-defined laboratory problems requiring hypothetical thinking and reasoning for their solution. By “well-defined” we mean that all information required to solve the problem according to the instructions is explicitly presented. For this reason, psychologists have traditionally regarded any influence of prior knowledge or belief about the problem content or context to be normatively irrelevant to the definition of a correct answer. Consequently, where such beliefs exert an influence this has often been termed a “bias” by the investigators concerned. The effects of prior belief, however, turn out to be so pervasive in these studies that reasoning researchers in the past decade or so have begun radically to reexamine their assumptions about the nature of rational reasoning. This reassessment has been no where more visible than in the study of deductive reasoning, one of the major paradigms in this field. Typical experiments involve presenting participants with the premises of logical arguments and asking them to evaluate a conclusion presented, or draw one of their own (for reviews, see Evans, Newstead, & Byrne, 1993; Manktelow, 1999). The deduction paradigm has its origins in logicism – the belief that logic provides the rational basis for human reasoning (Evans, 2000a). The modern study of deductive reasoning dates from the 1960s, where it was motivated by the writings of psychologists such as Henle (1962) and especially Jean Piaget (Inhelder & Piaget, 1958), who proposed that adult human reasoning was inherently logical.