So far I explained why modal empiricism is the best position on epistemological grounds:
- it can answer the no-miracle argument, while not falling prey to a pessimistic meta-induction (structural realism claims to do this as well, but epistemic structural realism is probably just empiricism--more on the ontic versions below);
- the inductive epistemology for modal statements on which it rests fits well with scientific practice, in particular the role of interventions in experimentation;
- it can account for explanations and causal discourse in science.
Three accounts of meaning
It is common nowadays, following the semantic view of theories, to think of scientific theories as a collection of models rather than a set of statements, but although models played an important role in the argument of the previous posts, I think the statement view is roughly correct, and roughly equivalent to the semantic view.
The semantic view was a sane reaction to the idealistic conceptions of logical empiricism. Its main import is that it emphasised the role models play in empirical confrontation, as mediators between the theory and experience. But as an attempt to bypass semantic questions, I think it is misguided, because models need to be expressed in a language to have a domain of application, and axioms unify various models in a coherent scheme, so the best way to express the content of a theory is with axioms expressed in a theoretical language.
Now comes the main question of the semantic side of scientific realism: how shall we interpret this theoretical language? There are roughly three options:
- theoretical terms are mere placeholders in the conceptual structure of the theory
- theoretical terms should be analysed by their intensional or extensional relations to our experience, or their function in experience
- essential realism:
- theoretical terms directly refer to natural properties and relations, or essences
Note that the question is not about what theories achieve to do, but about what they purports to do: about their meaning, or about what would make them true if they were true. The emphasis on linguistic aspects the logical empiricist entertained is not misguided. After all, meaning is generally analysed in terms of truth conditions: the meaning of a statement is what would make this statement true. There are three traditional theories of truth, which roughly correspond to our three options: coherentism, according to which truth is coherence with a conceptual scheme, pragmatism, according to which truth has to do with the ideal success of a statement, or our capacities to assess this statement, and correspondence, according to which truth is correspondence to reality. In sum, the locus of truth-conditions, and meaning, is either in our representations only, in experience, or in an independent reality. (One could add deflationist theories of truth but I forget them here because they do not really shed light on meaning.)
To many authors, only a correspondence theory can sustain genuine realism: the others would rather lead us to various forms of empiricism, relativism or idealism. Kripke's defence of essential realism has been influential, which is a reason why the focus in philosophy of science moved away from semantic aspects, toward epistemic aspects: given that our theories purports to refer to natural properties, or to correspond to reality, how would we know that they succeed? A correspondence theory of truth introduces a gap between the content of our representations and our capacities to know that they are true, and this is where realists resort to abductive reasoning to fill the gap. The fact that this is problematic is a good reason not to forget about other options, and to keep an eye on philosophy of language. Interestingly, pragmatics became a big subject there, with an emphasis on the role of intentional aspects and context on meaning, and direct reference is not uncontroversial anyway.
This account seems unfair to structuralists, because structural realists would typically accept a correspondence theory of truth, but apply it to the structure directly. In a sense, this is a way to accommodate coherentist aspects (for the vocabulary) with a correspondence truth (for the structure). Now I'm not sure it works: Putnam's model-theoretic argument is a good argument against this kind of view. Structural realism is unstable. It must tell us what the relata of the structure are. If they're elements of experience, this is just empiricism. If they're natural properties, this is just standard realism. If they're identified by the structure only, this is a mathematical platonism, not so far from idealism, and this is vacuous as a realism (because any mathematical structure exists, abstractly: no big deal). So, basically, we fall back on one of our three options.
What about modal empiricism?
Well, a first remark is that I defended that modal empiricism is not realism, because it rests on an inductive, not abductive epistemology. But the contrast between empiricism and realism is not initially a contrast in which inferences are valid or not, but in whether one holds our theories to be true, or merely empirically adequate. Could an induction on possible situations be enough to know that our theories are true after all? Couldn't modal empirical adequacy collapse to truth?
Another remark is that modal empiricism does not entertain a distinction between what is observable or not. Empirical adequacy is expressed in terms of application and predictions, which concern the objectivable aspects of situations. I emphasised how active intervention was necessary to test theories when they posit unobservable entities, such as proteins. On the surface, the arguments are similar to the arguments in favour of entity realism. Again, couldn't modal empiricism be enough to claim that these entities exist? If these entities are identified by their causal role (functionally that is) and if causal relations can be empirically assessed, they certainly exist for the modal empiricist.
Finally, note that essences and intensions are often analysed in modal terms. If interpreting a theory amounts to describe the essential properties it refers to, or to give it an intension, and if, as I have argued, modal statements are not underdetermined by experience, then perhaps the interpretation of a theory is not underdetermined either?
Take Quine's example of renates (creatures with kidneys) and cordates (creatures with a heart). Both have the same extension (all animals with a kidneys have a heart) but a different intension. We could take this intension as a description of their essence. Imagine a theory that says that renates are hairy and another that says that cordates are hairy. For a standard empiricist, both have the same empirical consequences, although they're different theories, but not so for a modal empiricist: we could intervene in the world to differentiate them (through genetic manipulations, say: create a cordate that is not are renate). But then, the interpretation of the theory matters for possible predictions, and we might as well be realists. Ok, but what if we discover that the manipulations are impossible? What if the genes that code for kidneys also code for hearts? Well, we'd have discover that renates and cordates are identical, as a matter of natural necessity. But we can still be realists, and claim to have discover the essence of cordates and renates. Again, the interpretation matters.
All this works so long as our theoretical terms somehow keep in touch with empirical observations. But what linguistic resources do we have, beyond experience and modalities, to interpret our theories? So it seems that modal empiricism is just standard realism.
Pragmatic truth and internal realism
The answer, I think, lies in the theory of truth one adopts, and incidentally, on the modalities involved. Correspondence truth introduces a gap between our epistemic abilities and the truth of our theory. But for the empiricist, no correspondence truth is involved. Theoretical terms are always interpreted in how they relate to experience. They matter only insofar as they change the conditions of application and prediction of theoretical models. We are talking about possible conditions here, and modal relations are involved, but no metaphysical necessity or possible worlds: only physical necessity, possible situations in the actual world, i.e. situations that we could implement by intervention.
This is some kind of reductionist account of meaning associated with a pragmatist theory of truth: following pragmatist truth, saying that our theories are modally adequate, or saying that they're true makes no practical difference.
There is an important difference with essential realism, which is that the meaning of theoretical terms will change from theory to theory, contra Kripke. Here, a theoretical term could be ideally analysed in terms of its modal relations to conditions of application and good prediction for the theory, i.e. in terms of its functional role in experimentation. This account seems similar to logical empiricists verificationism, or to operationalism, but the modal aspect and the pragmatic aspect of this construal (in particular, the fact that it does not rest on a distinction between observable and unobservable, and that the notion of application can incorporate pragmatic or contextual aspects) can help overcome the difficulties of these standard positions (such as the reduction of dispositional terms). But the problem of theoretical change remains: observations and interventions are theory-laden, so can't a new theory change the conditions of application of our theoretical terms? Then Kripke's arguments would apply: generally, we are willing to say that we could discover that gold is not yellow (because this is an illusion for example). Gold is not identical with its manifestations: it is what causes these manifestations. Similarly, we would be willing to accept that the way we apply terms like "electron" in experiments is misguided, in light of a new theory. The term "electron" cannot be analytically synonymous with some conditions of application, even extending to possible conditions of application.
I don't think this is a problem if we reject a strict notion of analyticity: indeed, the meaning of theoretical terms will change from theory to theory. However, experimental practices generally survive theory change. We can measure temperature with a thermometer, whether we endorse classical thermodynamics or statistical physics. The reason for this is a continuity in empirical adequacy. So, precisely, modal empiricism has the resources to answer Kripke's arguments: we keep using the same terms because they approximately play the same causal role, and we can be confident that they will continue to do so because our theories are modally adequate. The meaning of "mass" changes from Newtonian gravitation to general relativity, and the rest mass we attribute to the sun changes slightly, but not drastically. If it were to change drastically, i.e. if we discovered that our theories are not modally adequate, well, we would stop using the term "mass" because it would not apply any more. So modal empiricism is not analytic, but still, it guarantee a pragmatic continuity in theoretical term use.
By the way, this is a nice way to associate intensional and essentialist aspects: if experimentation is theory laden, then the intension of theoretical terms will tend to fixate on projectible predicates, i.e. to "group" objectivable aspects in such a way that necessary relations can be attributed to these groups. They mimic essential properties, because the "analytical" necessity associated with their intension (and the structure of the theory) will tend to match physical necessity. Or in a Quinean vein, there is no strict distinction between analytic and synthetic necessity, and meaning itself can be discovered empirically.
The conclusion of this is that modal empiricism, equipped with a pragmatic theory of truth, is just realism: not genuine realism for the aficionados of correspondence truth, but, at least, pragmatic realism (or perhaps internal realism).