THE HISTORICAL THESIS OF EVIDENCE

According to a standard view, predictions of new phenomena provide stronger evidence for a theory than explanations of old ones. More guard­edly, a theory that predicts phenomena that did not prompt the initial formulation of that theory is better supported by those phenomena than is a theory by known phenomena that generated the theory in the first place.

Stephen Brush (1989) takes issue with this on historical grounds. He ar­gues that generally speaking scientists do not regard the fact that a theory predicts new phenomena, even ones of a kind totally different from those that prompted the theory in the first place, as providing better evidential support for that theory than is provided by already known facts explained by the theory. By contrast, Brush claims, there are cases, including general relativity and the periodic law of elements, in which scientists tend to consider known phenomena explained by a theory as constituting much stronger support than novel predictions.^[87]

Both the predictionist and the explanationist are committed to an in­teresting historical thesis about evidence, namely:

Historical thesis: Whether some claim e, if true, is evidence for an hypothesis h, or how strong that evidence is, depends on certain historical facts about e, h, or their relationship.

For example, whether, or the extent to which, e counts as evidence for h depends on whether e was known before or after h was formulated. Var­ious historical positions are possible, as Alan Musgrave (1974) noted years ago in a very interesting article. On a simple predictionist view (which Musgrave classifies as “purely temporal”) e supports h only if e was not known when h was first proposed.

Is the historical thesis true or false? I propose to argue that it is some­times true, and sometimes false, depending on the type of evidence in question. Then I will consider what implications, if any, this has for the debate between Brush and the predictionists.

Before beginning, however, let me mention a curious but interesting fact about various well-known philosophical theories or definitions of ev­idence. As Laura Snyder (1994) points out in a perceptive paper entitled “Is Evidence Historical?” most such theories, including Carnap's (1962) a priori theory of confirmation, Hempel's (1945) satisfaction theory, Glymour's (1980) bootstrap account, and the usual hypothetico-deductive account, are incompatible with the historical thesis. They hold that whether, or the extent to which, e is evidence for, or confirms, h is an objective fact about e, h, and their relationship. It is in no way affected by the time at which h was first proposed, or e was first known, or by the intentions with which h was formulated. Defenders of these views must reject both the predictionist and the explanationist claims about evidence. They must say that whether, or the extent to which, e supports h has nothing to do with whether e was first formulated as a novel pre­diction from h or whether e was known before h and h was constructed to explain it.

Accordingly, we have two extreme or absolutist positions. There is the position, reflected in the historical thesis, that evidence is always historical (in the sense indicated). And there is a contrasting posi­tion, reflected in certain standard views, that evidence is never histor­ical. Does the truth lie at either extreme? Or is it somewhere in the middle?

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