Theories

Popper's Account and Theories as Explanations

Theories were thought at this time to be constructed on the basis of evidence through logical induction—generalizing from observed instances to formulate universal claims. This set up the Problem of Induction, which David Hume had first formulated in the latter part of the 18th century, and Karl Popper revived as a problem in 1934,in his Logik der Forschung (“the logic of research,” unfortunately titled The Logic of Scientific Discovery when published in English). Popper observed, as Hume had, that a universal claim cannot be derived from any finite number of observations and so cannot be rationally justified or verified, as the positivists had sought to do. He defined theories as “universal statements…nets cast to catch what we call ‘the world’: to rationalize, to explain, and to master it.” Theories explained events by giving a deduction from their universal laws and singular initial conditions that described the event to be explained. Carl Hempel generalized this as the nomological-deductive model of explanation. Variations for law-like generalizations that were not exceptionless universal statements, and for stochastic and probabilistic inferences, were also developed by Hempel and others. In particular, a historical form of explanation worked, so to speak, in reverse: if you could produce a law-like generalization from which, together with some initial and boundary terms, you could deduce (or induce or infer in some manner) the events that were recorded, then you had offered an explanation of the events—the so-called covering law model. If there were more than one generalization that covered the event, then the best theory was the one that covered the most events or cases. Prediction was not possible in historical science, of course, but retrodiction or the “prediction” of already known events from the covering law explanation was considered to be equivalent logically to prediction in the experimental and present sciences.

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