Lecture 17

4/5/94

Scientific Realism Vs. Constructive Empiricism

Today we begin to discuss a brand new topic, i.e., the debate between scientific realism and constructive empiricism. This debate was provoked primarily by the work of Bas van Fraassen, whose critique of scientific realism and defense of a viable alternative, which he called constructive empiricism, first reached a wide audience among philosophers with the publication of his 1980 book The Scientific Image. Today we will discuss (1) what scientific realism is, (2) what alternatives are available to scientific realism, specifically van Fraassen's constructive empiricism.

What Is Scientific Realism?

Scientific realism offers a certain characterization of what a scientific theory is, and what it means to "accept" a scientific theory. A scientific realist holds that (1) science aims to give us, in its theories, a literally true story of what the world is like, and that (2) acceptance of a scientific theory involves the belief that it is true.

Let us clarify these two points. With regard to the first point, the "aims of science" are to be distinguished from the motives that individual scientists have for developing scientific theories. Individual scientists are motivated by many diverse things when they develop theories, such as fame or respect, getting a government grant, and so on. The aims of the scientific enterprise are determined by what counts as success among members of the scientific community, taken as a whole. (Van Fraassen's analogy: The motives an individual may have for playing chess can differ from what counts as success in the game, i.e., putting your opponent's king in checkmate.) In other words, to count as fully successful a scientific theory must provide us with a literally true description of what the world is like.

Turning to the second point, realists are not so naive as to think that scientists' attitudes towards even the best of the current crop of scientific theories should be characterized as simple belief in their truth. After all, even the most cursory examination of the history of science would reveal that scientific theories come and go; moreover, scientists often have positive reason to think that current theories will be superseded, since they themselves are actively working towards that end. (Example: The current pursuit of a unified field theory, or "theory of everything.") Since acceptance of our current theories is tentative, realists, who identify acceptance of a theory with belief in its truth, would readily admit that scientists at most tentatively believe that our best theories are true. To say that a scientist's belief in a theory is "tentative" is of course ambiguous: it could mean either that the scientist is somewhat confident, but not fully confident, that the theory is true; or it could mean that the scientist is fully confident that the theory is approximately true. To make things definite, we will understand "tentative" belief in the former way, as less-than-full confidence in the truth of the theory.

Constructive Empiricism: An Alternative To Scientific Realism

There are two basic alternatives to scientific realism, i.e., two different types of scientific anti-realism. That is because scientific realism as just described asserts two things, that scientific theories (1) should be understood as literal descriptions of the what the world is like, and (2) so construed, a successful scientific theory is one that is true. Thus, a scientific anti-realist could deny either that theories ought to be construed literally, or that theories construed literally have to be true to be successful. A "literal" understanding of a scientific theory is to be contrasted with understanding it as a metaphor, or as having a different meaning from what its surface appearance would indicate. (For example, some people have held that statements about unobservable entities can be understood as nothing more than veiled references to what we would observe under various conditions: e.g., the meaning of a theoretical term such as "electron" is exhausted by its "operational definition.") Van Fraassen is an anti-realist of the second sort: he agrees with the realist that scientific theories ought to be construed literally, but disagrees with them when he asserts that a scientific theory does not have to be true to be successful.

Van Fraassen espouses a version of anti-realism that he calls "constructive empiricism." This view holds that (1) science aims to give us theories that are empirically adequate, and (2) acceptance of a scientific theory involves the belief that it is empirically adequate. (As was the case above, one can tentatively accept a scientific theory by tentatively believing that the theory is empirically adequate.) A scientific theory is "empirically adequate" if it gets things right about the observable phenomena in nature. Phenomena are "observable" if they could be observed by appropriately placed beings with sensory abilities similar to those characteristic of human beings. On this construal, many things that human beings never have observed or ever will observe count as "observable." On this understanding of "observable," to accept a scientific theory is to believe that it gets things right not only about the empirical observations that scientists have already made, but also about any observations that human scientists could possibly make (past, present, and future) and any observations that could be made by appropriately placed beings with sensory abilities similar to those characteristic of human scientists.

The Notion Of Observability

Constructive empiricism requires a notion of "observability." Thus, it is important that we be as clear as possible about what this notion involves for van Fraassen. Van Fraassen holds two things about the notion of observability:

(1) Entities that exist in the world are the kinds of things that are observable or unobservable. There is no reason to think that language can be divided into theoretical and observational vocabularies, however. We may describe observable entities using highly theoretical language (e.g., VHF receiver," "mass," "element," and so on); this does not, however, mean that whether the things themselves (as opposed to how we describe or conceptualize them) are unobservable or not depends on what theories we accept. Thus, we must carefully distinguish between observing an entity from observing that an entity exists meeting such-and-such a description. The latter can be dependent upon theory, since descriptions of observable phenomena are often "theory-laden." However, it would be a confusion to conclude from this that the entity observed is a theoretical construct.

(2) The boundary between observable and unobservable entities is vague. There is a continuum from viewing something with glasses, to viewing it with a magnifying lens, with a low-power optical microscope, with a high-power optical microscope, to viewing it with an electron microscope. At what point should the smallest things visible using a particular instrument count as "observable?" Van Fraassen's answer is that "observable" is a vague predicate like "bald" or "tall." There are clear cases when a person is bald or not bald, tall or not tall, but there are also many cases in between where it is not clear on which side of the line the person falls. Similarly, though we are not able to draw a precise line that separates the observable from the unobservable, this doesn't mean that the notion has no content, since there are entities that clearly fall on one side or the other of the distinction (consider sub-atomic particles vs. chairs, elephants, planets, and galaxies). The content of the predicate "observable" is to be fixed relative to certain sensory abilities. What counts as "observable" for us is what could be observed by a suitably placed being with sensory abilities similar to those characteristic of human beings (or rather, the epistemic community to which we consider ourselves belonging). Thus, beings with electron microscopes in place of eyes do not count.

Arguments In Favor Of Scientific Realism: Inference To The Best Explanation

Now that we have set out in a preliminary way the two rival positions that we will consider during the next few weeks, let us examine the arguments that could be given in favor of scientific realism. An important argument that can be given for scientific realism is that we ought rationally to infer that the best explanation of what we observe is true. This is called "inference to the best explanation." The argument for this view is that in everyday life we reason according to the principle of inference to the best explanation, and so we should also reason this way in science. The best explanation, for example, for the fact that measuring Avogadro's number (a constant specifying the number of molecules in a mole of any given substance) using such diverse phenomena as Brownian motion, alpha decay, x-ray diffraction, electrolysis, and blackbody radiation gives the same result is that matter really is composed of the unobservable entities we call molecules. If it were not, wouldn't it be an utterly surprising coincidence that things behaved in very different circumstances exactly as if they were composed of molecules? This is the same kind of reasoning that justifies belief that an apartment has mice. If all the phenomena that have been observed are just as would be expected if a mouse were inhabiting the apartment, isn't it then reasonable to believe that there's a mouse, even though you've never actually seen it? If so, why should it be any different when you are reasoning about unobservable entities such as molecules?

Van Fraassen's response is that the scientific realist is assuming that we follow a rule that says we should infer the truth of the best explanation of what we have observed. This is what makes it look inconsistent for a person to insist that we ought not to infer that molecules exist while at the same time insisting that we ought to infer that there is a mouse in the apartment. Why not characterize the rule we are following differently--i.e., that we infer that the best explanation of what we observe is empirically adequate? If that were the case, we should believe in the existence of the mouse, but we should not believe anything more of the theory that matter is composed of molecules than that it adequately accounts for all observable phenomena. In other words, van Fraassen is arguing that, unless you already assume that scientists follow the rule of inference to the (truth of the) best explanation, you cannot provide any evidence that they follow that rule as opposed to following the rule of inference to the empirical adequacy of the best explanation.

We will continue the discussion of inference to the best explanation, as well as other arguments for scientific realism, next time.