Dynamics of Theory Change: The Role of Predictions

“What did the President know and when did he know it?” Senator Howard Baker, Watergate hearings, 1973 Why do scientists accept or reject theories? More specifically: why do they change from one theory to another? What is the role of empirical tests in the evaluation of theories? This paper focuses on a narrowly-defined question: in judging theories, do scientists give greater weight (other things being equal) to successful novel predictions than to successful deductions of previously-known facts? The affirmative answer is called the “predictivist thesis” (Maher 1988). It is primarily philosophers who are interested in this question, and they have treated it mostly as a normative or logical problem. Can the writings of historians of science tell us how scientists have treated novel predictions in the past? Until recently historians have rarely addressed this point.