Van Fraassen’s constructive empiricism is one of the most discussed current alternatives to scientific realism. Contrary to positivists, Van Fraassen does believe that scientific theories must be taken literally. In that sense, he rejects the positivist reinterpretations of scientific statements, according to which talk of unobservablesis only a convenient abbreviation of complicated talk about observables. For the positivists, in principle,everything scientists say about unobservable reality can be expressed, without any kind of loss whatsoever, into statements referring only to observables. But Van Fraassen disagrees. He thinks that scientific statements regarding unobservables are meaningful. If a theory says that ‘electrons are not planets’ then the theory is asserting the existence of both electrons and planets. And if the theory happens to be true, both electrons and planets would exist. But here is the twist in Van Fraassen’s story. According to him, “there is no need to believe good theories to be true, nor to believe ipso facto that the entities they postulate are real” (1065). He defines Constructive Empiricism like this:

“Science aims to give us theories which are empirically adequate; and acceptance of a theory involves a beliefonly that it isempirically adequate” (1065)

A theory is empirically adequate “if what it says about the observable things and eventsin the world is true –exactly if it ‘saves the phenomena’” (1065). Contrast this with what he takes to be the correct definition of Scientific Realism:

“Science aims to give us, in its theories, a literally true story of what the world is like; and acceptance of a scientific theory involves the belief that it is true” (1062)

The first thing to note is that both definitions focus on the aims of science, and not on its actual accomplishments. According to the constructive empiricist picture, science does posit unobservables, but its aim is only to be correct about its observational claims. Against this, the scientific realist picture says that the aim of science is to be actually correct in their claims about unobservables. By focusing merely on the aims of science, Van Fraassen in a way lowers the stakes of this debate. It is no longer about whether or not current scientific theories are correct regarding the unobservable part of the world, but about what is supposed to be the main goal of these theories. Now, whether or not this is a good way of framing the debate is debatable. Most realistsbelieve, I think, that science gets at least part of the unobservable world right. (Van Fraassen’s reply, as we saw in week 6, is that there is no noncircular way to verify the truth of claims about unobservables. But as we saw, the same happens with claims regarding observables!)

Now, something can be observable and yet unobserved. ‘Stones are hard’ aims to be true about both observed stonesand those that haven’t been. That statement, however, is not about ‘unobservables’, in the sense of objects that cannot be observed in principle by any human. ‘Stones are hard’ refers to the observable object stone and the observable property of being hard. It’s not about observations made at one point in time. Accepting a theory means accepting what it says about the observable part of the world, past, present, and future. Observable, on Van Fraassen’s view, is what we get in unaided acts of perception (1067).

“The human organism” says Van Fraassen, “is, from the point of view of physics, a certain kind of measuring apparatus. As such, it has certain inherent limitations… It is these limitations to which the ‘able’ in ‘observable’ refers—our limitations, qua human beings” (1070).

But with respect to the unobservable reality, an accepted theory may as well be false. It really doesn’t matter for the understanding of science. In that sense, constructive empiricism advocates agnosticism regarding the theory’s claims about the unobservable reality.

In terms of our discussion regarding scientific inference, we can say that Van Fraassen accepts ‘horizontal inferences’, or if you want ‘inferences that remain at the surface’, that is, inductive inferences from observable cases to observable cases. But he doesn’t accept ‘vertical inferences’, ‘inferences that go deeper than the surface’, that is, abductive inferences which conclusions refer to the unobservable causes of the phenomena.

Many realists have contested this distinctionas arbitrary. Why believingin induction, despite Hume, but not believing in abduction? There are many things that are unobserved but observable in Van Fraassen’s lights, like the core of the Earth, but it seems that our understanding of it is purely theoretical and also based on abductive inferences. There’s no reason to reject these inferences in those cases, but then if that’s true there wouldn’t be any reason to rejecting these inferences in other cases as well. The laws of nature go well beyond evidence, just like abduction does. Why would we prefer one over the other is not clear, despite Van Fraassen’s argument.

One important pragmatic aspect of Van Fraassen’s view, is that although accepting a theory only means accepting that the theory is empirically adequate, one can still use the whole machinery the theory puts at one’s disposal, which includes the unobservable structures and processes. Van Fraassen is not advocating for a change in the practice of science. His point is rather that if the theory works and accommodates past, present and future empirical evidence, then we don’t need to worry about whether the theory is true. ‘Why is the theory so successful?’ is not an interesting question. Perhaps this theory is successful because all other unsuccessful theories died out and were abandoned (this is a kind of ‘survival of the fittest’ argument). The reason our theories are successful is that we reject those that aren’t. A meta-scientific explanation regarding why they are successful is out of order, on Van Fraassen’s view. This has been contested by many philosophers who think that explaining the success of scienceis one of the most interesting tasks of philosophy of science, and there’s no reason to abandon it.

A final remark with respect to Van Fraassen’s criticism of the overlap argument. We saw that the overlap argument proposes some sort of instrument calibration. For example, we can verify the magnification powers of a microscope by applying it to observable objects first. Once we do that, there is no reason not to trust it when applied to things that would be unobservable to the naked eye, like proteins. Van Fraassen’s reliance on human capacities of observation seems arbitrary, because if we accept what we see through our spectacles, there’s no in-principle reason not to accept what we see through a microscope. Against this, Van Fraassen says that that is debatable, because the term observable is just vague, but there are clear cases of unobservableentities, like so-called unobservables in principle, and the overlap argument does not extend to them. As I mentioned before, it doesn’t seem that the overlap argument itself can deal with this objection, which doesn’t mean that antirealism wins, because as we will see, there are many more arguments defending this view.