Lecture 18


Inference To The Best Explanation As An Argument For Scientific Realism

Last time, I ended the lecture by alluding to an argument for scientific realism that proceeded from the premise that the reasoning rule of inference to the best explanation exists. Today, we will examine in greater detail how this argument would go, and we'll also discuss the notion of inference to the best explanation in greater detail.

The Reality Of Molecules: Converging Evidence

As I noted last time, what convinced many scientists at the beginning of this century of the atomic thesis (i.e., that matter is composed of atoms that combine into molecules, and so on) was that there are many independent experimental procedures all of which lead to the same determination of Avogadro's number. Let me mention some of the ways in which the number was determined.

(1) Brownian Motion. Jean Perrin studied the Brownian motion of small, microscopic particles known as colloids. (Brownian motion was first noted by Robert Brown, in the early 19th century.) Though visible only through a microscope, the particles were much larger than molecules. Perrin determined Avogadro's number from looking at how particles were distributed vertically when placed in colloidal suspensions. He prepared tiny spheres of gamboge, a resin, all of uniform size and density. He measured how the particles were distributed vertically when placed in water; calculating what forces would have to be in place to account for this keeping the particles suspended, he could calculate their average kinetic energy. If we know the mass and velocities, we can then determine the mass of a molecule of the fluid, and hence Avogadro's number, which is the molecular weight divided by the mass of a single molecule.

(2) Alpha Decay. Rutherford recognized that alpha particles were helium nuclei. Alpha particles can be detected by scintillation techniques. By counting the number of helium atoms that were required to make up a certain mass of helium, Rutherford calculated Avogadro's number.

(3) X-ray diffraction - A crystal will diffract x-rays, the matrix of atoms acting like a diffraction grating. From the wavelength of the x-rays and the diffraction pattern, you can calculate the spacing of the atoms. Since that is regular in a crystal, you could then determine how many atoms it takes to make up the crystal, and so Avogadro's number. (Friedrich & Knipping)

(4) Blackbody Radiation - Planck derived a formula for the law of blackbody radiation, which made use of Planck's constant (which was obtainable using Einstein's theory of the photoelectric effect) and macroscopically measurable variables such as the speed of light to derive Boltzmann's constant. Then you use the ideal gas law PV = nRT; n is the number of moles of an ideal gas, and R (the universal gas constant) is the gas constant per mole. Boltzmann's constant k is the gas constant per molecule. Hence R/k = Avogadro's number (number of molecules per mole).

(5) Electrochemistry - A Faraday F is the charge required to deposit a mole of monovalent metal during electrolysis. This means that you can calculate the number of molecules per mole if you know the charge of the electron, F/e = N. Millikan's experimental measurements of the charge of the electron can then be used to derive Avogadro's number.

Now, the scientific realist wants to claim that the fact that all of these different measurement techniques (along with many others not mentioned here) all lead to the same value for Avogadro's number. The argument, then, is how you can explain this remarkable convergence on the same result if it were not for the fact that there are atoms & molecules that are behaving as the theories say they do? Otherwise, it would be a miracle.

Inference To The Best Explanation (Revisited)

We have to now state carefully what is being claimed here. The scientific realist argues that the fact that the reality of molecules explains the convergence on Avogadro's number better than its rival, that the world is not really molecular but that everything behaves as if it were. That is because given the molecular hypothesis, convergence would be strongly favored, whereas if the underlying world were not molecular we wouldn't expect any stability in such a result. Now it is claimed that being the best explanation of something is a mark of truth. Thus, we have an inference pattern.

A explains X better than its rivals, B, C, and so on.

The ability of a hypothesis to explain something better
than all its rivals is a mark of its truth.

A is true.

Now why should we think that this is a good reasoning pattern? (That is, why should we think that the second premise is true?) The scientific realist argues that it is a reasoning pattern that we depend on in everyday life; we must assume the truth of the second premise if we are to act reasonably in everyday life. To refer to the example we discussed the last session, the scientific realist argues that if this reasoning pattern good for the detective work that infers the presence of an unseen mouse, it is good enough for the detective work that infers the presence of unseen constituents of matter.

Van Fraassen has argued that the hypothesis that we infer to the truth of our best explanation can be replaced by the hypothesis that we infer to the empirical adequacy of our best explanation without loss in the case of the mouse, since it is observable. How then, can you determine whether we ought to follow the former rule rather than the latter? The only reason we have for thinking that we follow IBE is everyday examples such as the one about mice; but the revised rule that van Fraassen suggests could account for this inferential behavior just as well as the IBE hypothesis. Thus, there is no real reason to think that we follow the rule of IBE in our reasoning.

Van Fraassen's Criticisms Of Inference To The Best Explanation

Van Fraassen also has positive criticisms of IBE. First, he argues that it is not what it claims to be. In science, you don't really choose the best overall explanation of the observable phenomena, but the best overall explanation that you have available to you. However, why should we think that the kinds of explanations that we happen to have thought of are the best hypotheses that could possibly be thought up by any intelligent being? Thus, the IBE rule has to be understood as inferring the truth of the best explanation that we have thought of. However, if that's the case our "best" explanation might very well be the best of a bad lot. To be committed to IBE, you have to hold that the hypotheses that we think of are more likely to be true than those that we do not, for that reason. This seems implausible.

Reactions On Behalf Of The Scientific Realist

(1) Privilege - Human beings are more likely to think up hypotheses that are true than those that are false. For otherwise, evolution would have weeded us out. False beliefs about the world make you less fit, in that you cannot predict and control your environment, and may likely die.

Objection: The kinds of things that selected us during our evolution based on our inferences do not depend on what we have believed to be true. They just have to not kill us, and enable us to have children. In addition, we only have to be able to infer what is empirically adequate.

(2) Forced Choice - We cannot do without inferring what goes beyond our evidence; thus the choice between competing hypotheses is forced. To guide our choices, we need rules of reasoning, and IBE fits the bill.

Objection - The situation may force us to choose the best we have; but it cannot force us to believe that the best we have is true. Having to choose a research program, for example, only means that you think that it is the best one available to you, and that you can best contribute to the advancement of science by choosing that one. It does not thereby commit you to belief in its truth.

The problem that is evinced here, and will crop up again, is that any rule of this sort has to make assumptions about the way the world is. The things that seem simple to us are most likely to be true; the things that seem to explain better than any of the alternatives that we've come up with are more likely to be true that those that do not; and so on.