A different strategy began by questioning the Hempelian proposal that ordinary explanations consist in explanation sketches whose force derives from an unarticulated ideal explanation. Philosophers such as Peter Achinstein and Bas van Fraassen offered pragmatic theories, according to which what counts as an explanation is contextually determined. Their accounts remained close to the everyday practice of explaining, but, to the extent that they eschewed context-independent conditions on explanation, they encouraged a return to the idea that explanation is a purely subjective business, a matter of what an audience will be satisfied with. Indeed, van Fraassen welcomed a conclusion of this type, holding that explanatory power is not an objective virtue of scientific theories.
The current state of scientific explanation is thus highly fragmentary. Although many philosophers hold that explanations trace causes, there is still considerable disagreement about whether or not the notion of causation should be analyzed and, if so, how. The question of whether theoretical explanation can always be construed in causal terms remains open. It is unclear whether unifying the phenomena is an explanatory virtue and how a satisfactory notion of unification should be understood. Perhaps most fundamentally, there are controversies about whether there is a single notion of explanation that applies to all sciences, all contexts, and all periods and about whether explanatory power counts as an objective quality of theories. Scientific laws
Similar uncertainties affect recent discussions of scientific laws. As already noted, logical empiricism faced a difficult problem in distinguishing between genuine laws and accidental generalizations. Just as theorists of explanation sometimes liberated themselves from hard problems by invoking a concept hitherto held as taboo—the notion of causation—so too some philosophers championed an idea of natural necessity and tried to characterize it as precisely as possible. Others, more sympathetic to Hume’s suspicions, continued the logical-empiricist project of analyzing the notion independently of the concept of natural necessity. The most important approach along these lines identifies the laws of nature as the generalizations that would figure in the best systematization of all natural phenomena. This suggestion fits naturally with the unificationist approach to explanation but encounters similar difficulties in articulating the idea of a “best systematization.” Perhaps more fundamentally, it is not obvious that the concept of “all natural phenomena” is coherent (or, even if it is, whether this is something in which science should be interested).
There is an even more basic issue. Why is the notion of a scientific law of any philosophical interest? Within the framework of logical empiricism, and specifically within Hempel’s approach to explanation, there was a clear answer. Explanations depend on laws, and the notion of law is to be explicated without appeal to suspect notions such as natural necessity. But Hempel’s approach is now defunct, and many contemporary philosophers are suspicious of the old suspicions, prepared to be more tolerant of appeals to causation and natural necessity. What function, then, would an account of laws now serve?
Perhaps the thought is that the search for the laws of nature is central to the scientific enterprise. But, to begin with, the scientific habit of labeling certain statements as “laws” seems extremely haphazard. There are areas, moreover, in which it is hard to find any laws—large tracts of the life and earth sciences, for example—and yet scientists in these areas are credited with the most important discoveries. James Watson and Francis Crick (1916–2004) won a Nobel Prize for one of the greatest scientific achievements of the 20th century (indeed, arguably the most fruitful), but it would be hard to state the law that they discovered. Accordingly, philosophers of science are beginning to abandon the notion that laws are central to science, focusing instead on the search for symmetries in physics, on the differing uses of approximate generalizations in biology, and on the deployment of models in numerous areas of the sciences.
Francis Crick.Print Collector/Heritage-Images/age fotostock
Scientific theories
The axiomatic conception
In similar fashion, contemporary philosophy of science is moving beyond the question of the structure of scientific theories. For a variety of reasons, that question was of enormous importance to the logical positivists and to the logical empiricists. Mathematical logic supplied a clear conception: a theory is a collection of statements (the axioms of the theory) and their deductive consequences. The logical positivists showed how this conception could be applied in scientific cases—one could axiomatize the theory of relativity, for example. Nor was the work of axiomatization an idle exercise, for the difficulties of formulating a precise criterion of cognitive significance (intended to separate good science from meaningless philosophical discussion) raised questions about the legitimacy of the special vocabulary that figures in scientific theories. Convinced that the sound and fury of German metaphysics—references to “Absolute Spirit” by Georg Wilhelm Friedrich Hegel (1770–1831) and talk of “the Nothing” by Martin Heidegger (1889–1976)—signified, indeed, nothing, logical positivists (and logical empiricists) recognized that they needed to show how terms such as electron and covalent bond were different.
They began from a distinction between two types of language. Observational language comprises all the terms that can be acquired by presentation of observable samples. Although they were skeptical about mixing psychology and philosophy, logical empiricists tacitly adopted a simple theory of learning: children can learn terms such as red by being shown appropriate swatches, hot by holding their hands under the right taps, and so forth. Logical empiricists denied that this observational vocabulary would suffice to define the special terms of theoretical science, the theoretical language that seemed to pick out unobservable entities and properties. Conceiving of theories as axiomatic systems, however, they drew a distinction between two types of axioms. Some axioms contain only theoretical vocabulary, while others contain both theoretical and observational terms. The latter, variously characterized as “correspondence rules” or “coordinating definitions,” relate the theoretical and observational vocabularies, and it is through them that theoretical terms acquire what meaning they have.
The last formulation blurs an important difference between two schools within logical empiricism. According to one school, the theoretical terms are “partially interpreted” by the correspondence rules, so, for example, if one such rule is that an electron produces a particular kind of track in a cloud chamber, then many possibilities for the meaning of the previously unfamiliar term electron are ruled out. A more radical school, instrumentalism, held that, strictly speaking, the theoretical vocabulary remains meaningless. Instrumentalists took scientific theories to be axiomatic systems only part of whose vocabulary—the observational language—is interpreted; the rest is a formal calculus whose purpose is to yield predictions couched in the observational vocabulary. Even instrumentalists, however, were able to maintain a distinction between serious theoretical science and the much-derided metaphysics, for their reconstructions of scientific theories would reveal the uninterpreted vocabulary as playing an important functional role (a result not to be expected in the metaphysical case).