Last time we looked at an argument for scientific realism based on an appeal to a putative rule of reasoning, called Inference to the Best Explanation (IBE). We examined a case study where the scientific realist argues that convergent but independent determinations of Avogadro's number were better explained by the truth of the molecular hypothesis than its empirical adequacy (Salmon 1984, Scientific Explanation and the Causal Structure of the World, pages 213-227). (Note that Salmon's treatment of these cases does not explicitly appeal to IBE, but to the Common Cause principle.) The primary problems with that argument were (1) the realist has given no reason to think that we as a rule infer the truth of the best explanation, rather than its empirical adequacy, and (2) it is impossible to argue for IBE as a justified rule of inference unless one assumes that human beings are, by nature, more likely to think up true explanations rather than ones that are merely empirically adequate. There are, of course, responses a realist can make to these objections, and we examined two responses to problem (2), one that argues that (a) evolution selected humans based on our ability to generate true rather than false hypotheses about the world, and the other that (b) accepts the objection but argues that we are somehow forced to believe the best available explanation. Neither of these responses seems very convincing (van Fraassen 1989, Laws and Symmetry, pages 142-150).
Today we will look at a moderate form of realism, which I will call "entity realism," and arguments for that view that do not depend on IBE. Entity realists hold that what one is rationally compelled to believe the existence of some of the unobservable entities postulated by our best scientific theories, but one is not obligated to believe that everything that our best theories say about those entities is true. Nancy Cartwright, for example, argues that we are compelled to believe in those entities that figure essentially in causal explanations of the observable phenomena, but not in the theoretical explanations that accompany them. The primary reason she gives is that causal explanations, e.g., that a change in pressure is caused by molecules impinging on the surface of a container with greater force after heat energy introduced into the container increases the mean kinetic energy of the molecules, make no sense unless you really think that molecules exist and behave roughly as described. Cartwright claims that you have offered no explanation at all if you give the preceding story and then add, "For all we know molecules might not really exist, and the world simply behaves as if they exist." Theoretical explanations, on the other hand, which merely derive the laws governing the behavior of those entities from more fundamental laws, are not necessary to believe, since a multiplicity of theoretical laws can account for the phenomenological laws that we derive from experiment. Cartwright argues that scientists use often different and incompatible theoretical models based on how useful those models are in particular experimental situations; if this is so, scientists cannot be committed to the truth of all their theoretical models. However, scientists do not admit incompatible causal explanations of the same phenomenon; according to Cartwright, that is because a causal explanation cannot explain at all unless the entities that play the causal roles in the explanation exist.
Cartwright's argument depends on a certain thesis about explanation (explanations can either cite causes or can be derivations from fundamental laws) and an associated inference rule (one cannot endorse a causal explanation of a phenomenon without believing in the existence of the entities that, according to the explanation, play a role in causing the phenomenon). As Cartwright sometimes puts it, she rejects the rule of Inference to the Best Explanation but accepts a rule of Inference to the Most Probable Cause. Van Fraassen, of course, is unlikely to acquiesce in such reasoning, since he rejects the notion that a causal explanation cannot be acceptable unless the entities it postulates exist; on the contrary, if what is requested in the circumstances is information about causal processes according to a particular scientific theory, it will be no less explanatory if we merely accept the theory (believe it to be empirically adequate) rather than believe that theory. Thus, the constructive empiricist can reject Cartwright's argument since he holds a different view of what scientific explanation consists in.
Hacking takes a different route in arguing to an entity realist position. Hacking argues that the mistake that Cartwright and van Fraassen both make is concentrating on scientific theory rather than experimental practice. His approach can be summed up in the slogans "Don't Just Peer, Interfere" (with regard to microscopes), and "If you can manipulate them, they must be real" (with regard to experimental devices that use microscopic particles such as electrons as tools). Let's look at his arguments for these two cases.
In his article, "Do We See Through a Microscope?" (Churchland and Hooker, eds., 1985, Images of Science), Hacking argues that what convinces experimentalists that they are seeing microscopic particles has nothing to do with the theory of those particles or of how a microscope behaves, but that they can manipulate those particles in very direct and tangible ways to achieve certain results.
Hacking's argument contains three elements, that (a) manipulation causes cognitive changes that give us new perceptual abilities, (b) we can manipulate the world in such a way as to create microstructures that have the same properties as macrostructures we can observe, and that, (c) combined with this fact, the convergence of the various instruments on the same visual results gives us additional reason to believe that what we are seeing is real, not an artifact of any particular instrument.
The final element (c) seems similar to the convergence argument we looked at last time, when we were discussing IBE. There is a difference, however, since what is at issue is not whether a single scientific theory implies things that are verified under many independent circumstances, but whether we are convinced that we are seeing something based on the fact of stable features using different viewing techniques. Nevertheless, it is an argument from coincidence--wouldn't it be a miracle if all these independent viewing techniques shared stable structural features and those features weren't really present in the microscopic specimen?--and stands or falls on the same grounds as were discussed last time.
However, that is not all that Hacking has at his disposal. His greatest strength is discussing how we acquire new modes of perception by using instruments to manipulate a world we cannot see. In his words, we don't see through a microscope, we see with a microscope. That is something that must be learned by interacting with the microscopic world, just as ordinary vision is acquired by interacting with the macroscopic world around us.
In addition, Hacking wants to argue that we come to manipulate things in ways that do not involve direct perception. This is where the example of using electrons to check for parity violation of weak, neutral currents comes in. In this case, Hacking argues that it might have once been the case that the explanatory virtues of atomic theory led one to believe in their existence; but now we have more direct evidence. We can now use electrons to achieve other results, and thus we are convinced of the existence of entities with well-defined, stable causal properties. That does not mean that we know everything there is to know about those particles (thus, we may disbelieve any of the particular theories of the electron that are in existence); however, that there are entities with certain causal properties is shown by experience, by manipulating electrons to achieve definite, predictable results. (Hence the slogan, "If you can manipulate them, they must be real.") This is why Hacking, like Cartwright, is an entity realist, but not a realist about scientific theories.
Next time, we will examine various responses that a constructive empiricist such as van Fraassen might give to such arguments.