Scientific realism is a philosophical view about science that consists of three theses:
- The metaphysical thesis: The world has a definite and mind-independent structure.
- The semantic thesis: Scientific theories should be taken at face value. They are truth-conditioned descriptions of their intended domain, both observable and unobservable. Hence, they are capable of being true or false. The theoretical terms featured in theories have putative factual reference.
- The epistemic thesis: Mature and predictively successful scientific theories are well confirmed and (approximately) true of the world. So the entities posited by them, or entities very similar to those posited, inhabit the world.
Let us call the first thesis of scientific realism metaphysical realism. What exactly is involved in the claim of mindindependence? One way to construe the opposite claim that the world is mind-dependent, along the lines of traditional idealism and phenomenalism, is to argue that the world consists of mental entities, be they ideas or actual and possible sense-data. Thus understood, minddependence is a thesis about the kind of stuff that makes up the world.
The insistence of scientific realism on metaphysical realism might be thought of as opposing this idealist or phenomenalist doctrine. It might be seen as a declaration that there is nonmental stuff in the world and, in particular, that the entities posited by scientific theories are material. This view is certainly part of the realist construal of mind-independence, but there is more.
There is another, more complicated and interesting, way to construe the claim that the world is mind-dependent. This way centers not on what types of entity exist (whether they are material or mental or what have you) but rather on what is involved in claiming that they exist.
There is a long antirealist philosophical tradition according to which it does not make sense to assert the existence (or reality) of some entities unless we understand this assertion to mean that ... , where the ellipsis is filled with a suitable epistemic/conceptual condition.
Much like realism, these views (call them varieties of verificationist antirealism) oppose idealism and phenomenalism. They entail the position (or at least are consistent with the claim) that material objects are real (be they the middlesized entities of common sense or unobservable entities).
The substantive disagreement between this antirealist tradition and realism is the sense of existence.Verificationist antirealism makes the world (or a set of entities) mind-dependent in a more sophisticated sense: What there is in the world is determined by what can be known to exist (verified to exist, rationally accepted as existing, or the like).
Hence it forges a logical-conceptual link between what there is in the world and what is affirmed as existing on the basis that it satisfies suitable epistemic conditions. Accordingly, the realist claim of mind-independence should be understood as logical or conceptual independence: What the world is like does not logically or conceptually depend on the epistemic means and conceptualizations used to get to know it.
Scientific realism allows for the possibility of a divergence between what there is in the world and what is issued as existing by a suitable set of conceptualizations and epistemic conditions. Verificationist antirealism precludes this possibility of divergence a priori by advancing an epistemic conception of truth.
No matter what the details of this conception are, the key idea is that truth is conceptually linked with epistemic conditions so tightly that a theory cannot be false even though epistemically justified (because it meets the relevant epistemic condition, for example, being under ideal circumstances theoretically justified or warrantedly assertable). Typically, realists honor the possibility of divergence by adopting a non-epistemic conception of truth (the standard candidate for which is the correspondence theory of truth).
Why should scientific realism incorporate the claim of mind-independence? Why, that is, cannot someone who accepts the reality of unobservable entities but regards them as mind-dependent (in the above sense) be a scientific realist? Ultimately at stake in the debate over scientific realism is a robust sense of objectivity, that is, a conception of the world as the arbiter of our changing and evolving conceptualizations of it. Scientific realism honors this conception by claiming that the world is mind-independent.
The kernel of its metaphysical thesis is that science is in the business of discovering what a world that is not of our making is like. This thesis implies that if the natural kinds posited by theories exist at all, they exist objectively, that is, independently of our ability to be in a position to know them, verify them, recognize them, etc., and hence that natural kinds, if anything, make scientific theories true.
This robust sense of objectivity contradicts verificationist antirealism. It also blocks a number of projectivist or social constructivist views about science from being realist. In the view of scientific realism, scientific theories and scientific theorizing in general, instead of projecting (or worse, socially constructing) the structure of the world, discover and map out an already structured, mind-independent world.
Let us call the second thesis of scientific realism, the view that scientific theories should be taken at face-value, semantic realism. This view too was motivated by problems with verificationism.
Verificationism, at least in its traditional form as defended by the logical positivists, runs together two separate issues: the evidential basis for the truth of an assertion and the semantic relation of reference or denotation. It thereby conflates the issue of what constitutes evidence for the truth of an assertion with the issue of what makes the assertion true.
This conflation was the product of concerns about the meaning of theoretical terms. Some empiricists thought that since the meaning of theoretical terms is not given directly in experience, these terms are semantically suspect. Hence, empiricists (even hard-core positivists like Ernst Mach) sought to show that theoretical statements and terms are parasitic on observational statements and terms.
This line of thought led to reductive empiricism, which treats theoretical statements as being disguised talk about observables and their actual (and possible) behavior. Interestingly, this view is consistent with the claim that theoretical statements have truth-values, but it understands their truth-conditions reductively: Their truthconditions can be fully captured in an observational vocabulary.
Hence, theoretical statements are ontologically innocuous: They do not refer to unobservable entities, and so imply no commitments to unobservable entities. Despite the heroic efforts of many empiricists (including the early Rudolf Carnap), all attempts to translate theoretical terms into observational terms have patently failed. As a result, empiricism became liberal. It admitted that theoretical terms and statements have excess content that cannot be fully captured by any reference to observable entities and phenomena.
If evidence-conditions and truth-conditions are kept apart, verificationism loses its bite. Semantic realism, simply put, says that there should not be two semantic standards, one for observational statements and another for theoretical ones. Observational statements, as well as theoretical statements, are true if and only if their truth-conditions obtain. Hence, theoretical terms, no less than observational terms, have putative factual reference.
If theoretical statements cannot be given truth-conditions in an ontology that dispenses with theoretical entities, a full and just explication of scientific theories simply requires commitment to irreducible unobservable entities, no less than it requires commitment to observable entities.
Instrumentalism claims that theories should be seen as (useful) instruments for organizing, classifying, and predicting observable phenomena. So the “cash value” of scientific theories is fully captured by what theories say about the observable world.
Faced with the semantic realist challenge that theoretical assertions are meaningful and purport to describe an unobservable reality, instrumentalism took refuge in Craig’s theorem and claimed that theoretical commitments in science are dispensable: Theoretical terms can be eliminated en bloc without loss in the deductive connections between the observable consequences of the theory.
If this is so, then the very question of whether theoretical terms can refer to unobservable entities evaporates. This challenge led Carl Hempel (1958) to formulate what he called “the theoretician’s dilemma.”
If the theoretical terms and the theoretical principles of a theory do not serve their purpose of a deductive systematization of the empirical consequences of a theory, then they are dispensable (unnecessary). But by Craig’s theorem, even if they do serve their purpose, they can still be dispensed with. Hence, the theoretical terms and principles of any theory are dispensable.
Is the theoretician’s dilemma compelling? Note first that the very idea of this dilemma rests on a sharp distinction between theoretical terms and observational ones. This dichotomy was severely challenged in the 1960s, when Pierre Duhem’s view that all observation is theory-laden resurfaced.
Along with it came the view that, strictly speaking, there are no observational terms. But even if the dichotomy is accepted, instrumentalism based on Craig’s theorem collapses. It is implausible to think of theories as establishing only a deductive systematization of observable phenomena.
Theories also offer inductive systematizations in the sense that theories can be used to establish inductive connections among observable phenomena: They function as premises in inductive arguments and, together with other premises concerning observable phenomena, yield conclusions that refer to observable phenomena. Seen as aiming to establish inductive connections among observables, theories are indispensable.
There followed a battery of indispensability arguments, fostered by Sellars (1963) and Quine (1960) among others, suggesting that theoretical terms are indispensable in any attempt to formulate a powerful and efficacious system of laws and to explain why observable entities obey the empirical laws they do.
Semantic realism opposes both instrumentalism and reductive empiricism. It renders scientific realism an “ontologically inflationary” view. Understood realistically, theories admit of a literal interpretation, that is, an interpretation according to which the world is populated by a host of unobservable entities and processes.
Semantic realism is not contested any more. All sides of the debate take theoretical discourse to be irreducible and contentful. It should be clear from the above discussion, however, that making semantic realism the object of philosophical consensus was no trivial feat.