In the 1990s, however, the controversy about the reality of unobservables was revived through the development of sophisticated antirealist arguments. Van Fraassen advocated a position that he called “constructive empiricism,” a view intended to capture the insights of logical empiricism while avoiding its defects. A champion of the semantic conception of theories, he proposed that scientists build models that are designed to “save the phenomena” by yielding correct predictions about observables. To adopt the models is simply to suppose that observable events and states of affairs are as if the models were true, but there is no need to commit oneself to the existence of the unobservable entities and processes that figure in the models. Rather, one should remain agnostic. Because the aim of science is to achieve correct predictions about observables, there is no need to assume the extra risks involved in commitment to the existence of unobservables.
A different antirealist argument, presented by Laudan, attacks directly the “ultimate argument” for realism. Laudan reflected on the history of science and considered all the past theories that were once counted as outstandingly successful. He offered a list of outmoded theories, claiming that all enjoyed successes and noting that not only is each now viewed as false, but each also contains theoretical vocabulary that is now recognized as picking out nothing at all in nature. If so many scientists of past generations judged their theories to be successful and, on that basis, concluded that they were true, and if, by current lights, they were all wrong, how can it be supposed that the contemporary situation is different—that, when contemporary scientists gesture at apparent successes and infer to the approximate truth of their theories, they are correct? Laudan formulated a “pessimistic induction on the history of science,” generalizing from the fact that large numbers of past successful theories have proved false to the conclusion that successful contemporary theories are also incorrect.
A third antirealist objection, formulated by both Laudan and Arthur Fine, charges that the popular defenses of realism beg the question. Realists try to convince their opponents by suggesting that only a realist view of unobservables will explain the success of science. In doing so, however, they presuppose that the fact that a certain doctrine has explanatory power provides a reason to accept it. But the point of many antirealist arguments is that allegations about explanatory power have no bearing on questions of truth. Antirealists are unpersuaded when it is suggested, for example, that a hypothesis about atoms should be accepted because it explains observable chemical phenomena. They will be equally unmoved when they are told that a philosophical hypothesis (the hypothesis of scientific realism) should be accepted because it explains the success of science. In both instances, they want to know why the features of the hypotheses to which realists draw attention—the ability of those hypotheses to generate correct conclusions about observable matters—should be taken as indicators of the truth of the hypotheses. “Piecemeal” realism
Realists tried to respond to these powerful points. One popular rejoinder is that antirealists cannot account for important facets of scientific practice. Thus, it is sometimes suggested that the routine method of conjoining theoretical claims from different scientific theories (as, for example, when earth scientists draw on parts of physics and chemistry) would not make sense unless there was a serious commitment to the approximate truth of the theoretical principles. Alternatively, one may take the practice of choosing certain kinds of experiments (experiments taken to be particularly revealing) to reflect a belief in the reality of underlying entities; thus, a medical researcher might choose a particular class of animals to inject with an antibiotic on the grounds that the concentration of bacteria in those animals is likely to be especially high.
Or the realist can attempt to argue that the kinds of inferences that the antirealist will acknowledge as unproblematic—for example, the generalization from observed samples to conclusions about a broader population of observable things—can be made only in light of an understanding of unobservable entities and mechanisms. One cannot tell what makes a sample suitable for generalization unless one has views about the ways in which that sample might be biased, and that will typically entail beliefs about relevant unobservable causes. Antirealists must either show that they have the resources to make sense of these and other features of scientific practice or offer reasons for thinking that the procedures in question should be revised.
Laudan’s pessimistic induction on the history of science attracted considerable scrutiny. Realists pointed out, correctly, that his list of successful past theories contains a number of dubious entries. Thus, it would be hard to defend the medieval theory of disease as caused by an imbalance of humours as particularly successful, and similar judgments apply to the geological catastrophism of the 18th century and the phlogiston theory of chemical combination.
Yet it is impossible to dismiss all of Laudan’s examples. One of his most telling points is that the account of the wave propagation of light of Augustin-Jean Fresnel (1788–1827) was spectacularly successful in explaining and predicting facts about diffraction and interference; one of its most dramatic successes, for example, was the prediction of the Poisson bright spot, a point of light at the centre of the shadow of a small rotating disk. (Ironically, the French mathematician for whom the spot is named, Siméon-Denis Poisson [1781–1840], believed that Fresnel was wrong and that the prediction of the spot was an absurd consequence of a false theory.) Fresnel, however, based his theory on the hypothesis that light waves are propagated in an all-pervading ether. Since contemporary science rejects the ether, it must also reject Fresnel’s theory as false.
This example is especially instructive, because it points to a refinement of realism. Contemporary optics takes over Fresnel’s mathematical treatment of wave propagation but denies the need for any medium in which the propagation takes place. So part of his theory is honoured as approximately correct, while the rest is seen as going astray because of Fresnel’s belief that any wave motion needs a medium in which the waves are propagated. Faced with a choice between saying that Fresnel’s theory is correct and saying that it is wrong, contemporary scientists would opt for the negative verdict. One would do greater justice to the situation, however, not by treating the theory as a whole but by judging some parts to be true and others false. Furthermore, when Fresnel’s work is analyzed in this way, it can be seen that the correct parts are responsible for its predictive successes. Appeals to the ether play no role when Fresnel is accounting for experimental data about interference bands and diffraction patterns. Hence, this example supports the realist linkage of success and truth by revealing that the parts of theory actually put to work in generating successful predictions continue to be counted as correct.
Indeed, realists can go farther than this: it can be argued that there is empirical evidence, of a kind that antirealists should be prepared to accept, of a connection between success and truth. People sometimes find themselves in situations in which their success at a particular task depends on their views about observable entities that they are temporarily unable to observe (think, for example, about card games in which players have to make judgments about cards that other players are holding). The evidence from such situations shows that systematic success is dependent on forming approximately correct hypotheses about the hidden things. There are no good grounds for thinking that the regularity breaks down when the entities in question lie below the threshold of human observation. Indeed, it would be a strange form of metaphysical hubris to suppose that the world is set up so that the connection between success and truth is finely tuned to the contingent perceptual powers of human beings.