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Many contemporary logicians acknowledge a plurality of logical theories and accept that theory choice is in part motivated by logical evidence. However, just as there is no agreement on logical theories, there is also no consensus on what constitutes logical evidence. In this paper, I outline Jean Piaget’s psychological theory of reasoning and show how he used it to diagnose and solve one of the paradoxes of material implication. I assess Piaget’s use of psychology as a source of evidence for logical theory in light of reservations raised by psychologism, and I highlight some ramifications for exceptionalism and anti-exceptionalism about logic by considering his use of psychology as logical evidence in the framework of genetic epistemology, Piaget’s research programme. I conclude that Piaget’s psychological theory of reasoning not only plausibly serves as a source of evidence for logical theory but also makes a strong case for anti-exceptionalism about logic.

Genetic epistemology analyzes the growth of knowledge both in the individual person (genetic psychology) and in the socio-historical realm (the history of science). But what the relationship is between the history of science and genetic psychology remains unclear. The biogenetic law that “ontogeny recapitulates phylogeny” is inadequate as a characterization of the relation. A critical examination of Piaget's Introduction a l'Epistemologie Genentique indicates these are several examples of what I call stage laws common to both areas. Furthermore, there is at least one example of a paradoxical inverse relation between the two — geometry. Both similarities and differences between the two domains require an explanation, a developmental explanation. Although such an explanation seems to be psychological in nature, it is not merely empirical but also normative (since psychology is both factual and normative according to Piaget). Hence genetic epistemology need not be reduced to psychology (narrowly conceived), but rather should be seen as being both empirical and normative and thus similar to certain types of contemporary philosophy of science.

In the wake of Frege’s attack on psychologism and the subsequent influence of Logical Positivism, psychological considerations in philosophy came to be viewed with suspicion. Philosophical questions, especially epistemological ones, were viewed as ‘logical’ questions, and logic was sharply separated from psychology. Various efforts have been made of late to reconnect epistemology with psychology. But there is little agreement about how such connections should be made, and doubts about the place of psychology within epistemology are still much in evidence. It therefore remains to be clarified just how such links should be established, and what impact they would have on the direction of epistemology.