“Is There a Valid Experimental Argument for Scientific Realism?” is reprinted by permission from The Journal of Philosophy 99 (2002), 470-495.

I define a concept of scientific realism employed by philosophical and scientific defenders and opponents of that doc­trine, and I argue against antirealists that Jean Perrin's experimental results with Brownian motion provide the basis for a valid empirical argument for scientific realism.

Long before atoms could be detected individually, scientists de­duced their existence from the way dust motes danced in drop­lets of liquid; atoms making up the liquid were colliding with and jostling the dust.

—New York Times¹

Wesley C. Salmon² claims that there is a valid experimental argument for scientific realism, and that one of the best examples is that provided by Jean Perrin³ early in the twentieth century. In 1908, Perrin conducted a series of experiments on Brownian motion on the basis of which he claimed that Avogadro's number N, the number of molecules in a substance whose weight in grams equals its molecular weight, is approximately 6 10²³. Per­rin drew the conclusion that unobservable molecules exist (pp. 213-227).

In memory of Wesley C. Salmon.

For very helpful questions and suggestions, I am indebted to Joseph Berkovitz, Sean Greenberg, Gregory Morgan, and Michael Williams.

1. March 29, 2001, p. A19.

2.Scientific Explanation and the Causal Structure of the World (Princeton, N.J: University Press, 1984), pp. 213-227.

3. “Brownian Movement and Molecular Reality,” reprinted in Mary Jo Nye, ed., The Question of the Atom (Los Angeles: Tomash, 1984), pp. 507-601 (see Annales de Chimie et de physique (1909)); and Atoms (Woodbridge, CT: Ox Bow, 1990).

By “scientific realism,” Salmon means a doctrine committed at least to the claim that unobservable entities exist. (What else, if anything, sci­entific realism does or should entail is controversial; the question will be taken up in sections 5-7).

I decided to try an empirical approach to the philosophical problem [of scien­tific realism]. Since it seemed unlikely that scientists would have been moved by the kinds of arguments supplied by philosophers, I felt that some insight might be gained if we were to consider the evidence and arguments that convinced scientists of the reality of unobservable entities. Although scientists, by and large, seem committed to the existence of a variety of unobservable entities..., the existence of atoms and molecules, as the microphysical constituents of ordinary matter, is the most clear and compelling example. (pp. 213-214)

Salmon proceeds to reconstruct Perrin's argument and to claim that it establishes the existence of molecules experimentally. It is a simple step to scientific realism:

Molecules exist.

Molecules are unobservable entities.

Therefore, unobservable entities exist.

Since the first step is itself the conclusion of an argument based on Perrin's experiments with Brownian motion, we seem to have an experimental argument for scientific realism. Because this argument convinced at least some antirealist scientists of the reality of molecules,^[111] Salmon challenges antirealist philosophers to say why they should not follow suit.

In what follows, I want to see how antirealist philosophers respond, or could respond, to Salmon's challenge to accept an experimental argument for scientific realism. First, however, I turn to Perrin's argument itself.

1.