Let us have the following:

(P1)For each theory H, there is always another theory  G such that G and H are empirically equivalent in the ‘weak sense’, (they are empirically equivalent in the weak sense if potential future observations may break this equivalence); (P2) if G and H are empirically equivalent in a weak sense, there is no reason to believe in H and not in G; (C) To contest this argument, it seems necessary to refer to extra-empirical reasons. It must attempt to show how, despite the empirical equivalence, there are other reasons for preferring one theory to another. For example, Popper proposed two preference criteria; he argued that G should be ignored if it were ‘ad hoc’ or did not imply predictions empirically falsifiable. A representative case is that the evolutionary theories are preferable to creationist ones.

Of course, both are compatible with the collected evidence (although the supporters of both parties argue that it is only his own) because they were built in compliance with it. Evolutionary theory, however, allowed to make some predictions (later proved correct): assume that men and chimpanzees are closely related species implied that morphologically intermediate species should be found and this is what happened with the discovery of Australopithecus. Nowadays the forecasts confirmed by the evolutionary theories are numerous and go beyond this simple example. By contrast, the creationist theories do not seem to have ever made falsifiable predictions and seem built ad hoc. Another way to build an ad hoc theory is the following:

G asserts that all what has been observed so far is compatible with H, but that the next observation will be different. In this case, what helps to choose in favor of H is induction. As past experience has never shown cases of sudden change of natural laws, inductively speaking we believe that the next observation will confirm H.

A simple argument to consider the possibility of underdetermination is that of the adjustment of the curve. It should be pointed out, incidentally, that in general we have the phenomenon of underdetermination when there is a situation where there is no true description, because information is not sufficient. The empirical underdetermination of scientific theories (due in part to Duhem and in part to Quine), according to which different theories can be compatible with the same set of observational data, result that the choice between theories would be based on pragmatic considerations of simplicity and comfort rather than on their capacity to provide a true representation of reality.

Let us return to the adjustment of the curve and assume that some scientists have derived experimentally a number of data that have been placed on a graph. If they had to draw the curve (function) that connects two quantities (for example), they will be inclined to use the one that best suits the results, even considering simplicity, elegance, consistency with other formulas and theories (what philosophers call ‘super-empiric virtues’). However, from a purely theoretical point of view, given a finite set of points (the data on the graph) there are infinite curves that pass by each of them. We call these curves or theories empirically equivalent in the weak sense.

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