In the previous post, I explained why, assuming that there is necessity in the world, relations of necessity are perfectly knowable on the basis of experience, without any recourse to abductive reasonning. That certainly works in favour of an endorsement of natural modalities for the empiricist: at least, epistemic arguments against it are not conclusive. But still, an empiricist could resist this kind of commitment by assuming that there is no necessity in the world from the start. She could interpret modal discourse in a pragmatic way, rather than assume that modal statements have truth values. In this post, I want to explain why she shouldn't.
As I said in the previous post, I don't think that the existence of necessity in the world itself (as opposed to relations of necessity if there is necessity in the world) can be confirmed or disconfirmed by experience. However, no position really comes without some general metaphysical framework, and this includes all empiricist positions: the idea that all knowledge comes from experience is not itself confirmed or disconfirmed by experience. The idea that our theories will continue to be empirically adequate in the future is not confirmed or disconfirmed by experience. The empiricist is only willing to assume the minimum necessary, and to refrain from speculations. One could frame this as some kind of transcendental argument: we need some basic assumptions to make sense of the world, but we shouldn't assume more.
This is exemplified by van Fraassen's defense of constructive empiricism. He does not say that his position is itself confirmed by experience, but he rephrases empiricism as a position about the aim of science, as a collective endeavour: its aim, according to van Fraassen, is to produce empirically adequate theories (not true theories). A scientist can well be a realist, but needs not be to be a good scientist. However, a scientist must at least assume that our theories are empirically adequate, and will continue to be in the future. Now if one thinks that scientific practice is a rational activity, one should also believe that our best theories are empirically adequate. This is a minimum.
I think this is the correct way of thinking about epistemological positions. But I think that it supports modal empiricism rather than constructive empiricism.