A Method of “Physical Speculation”

“What to Do if You Want to Defend a Theory You Can't Prove” is reprinted by permission from The Journal of Philosophy, January 2010. I formulate a method employed by James Clerk Maxwell in 1875 to argue for the truth of molecular theory without any experi­mental proof that molecules exist.

In 1875 the theoretical physicist James Clerk Maxwell published a paper in Nature entitled “On the Dynamical Evidence of the Molecular Constitution of Bodies.”^[CXCVI] In it he argues for the existence of molecules and for various claims of the molecular theory that he and others had been developing, including that molecules satisfy dynamical principles of classical physics. He does so without any experimental proof for his fundamental claims. Since he regards this as contrary to a prominent methodological view that the defense of a theory requires experimental proof, at the outset he announces that he will employ a different scientific method. It is designed for developing and defending theories that postu­late objects that, at the time, cannot be observed, and that make claims about such objects that, at the time, cannot be demonstrated to be true by observation and experiment.

Two questions are of special interest to Maxwell. First, can you use the method to develop and defend a theory about unobservables in a way that can make it possible to be justified in believing the theory (or at least the set of its central and distinctive assumptions) to be true, without being

able to experimentally prove that it is true?^[197] Second, can you do so in a way that is sufficiently precise and complete to answer a range of questions about the unobservables postulated, even if you have no epistemic warrant for some of these answers? Maxwell gives an affirmative answer to both questions. He refers to his method as a “method of physical speculation.” He takes it to be different from an inductive method of a sort espoused by Newton and Mill, which he regards as too demanding for his purposes, and from “the method of hypothesis” (or hypothetico-deductivism), which he deems too weak.

Maxwell gives only a very brief general description of his method, leaving his readers the task of understanding what it is from seeing how he actually employs it in defending his molecular theory. Whether it is worthy of being called a “method” at all, or just a general strategy, or something else, I believe that it is important for philosophers to consider. It is, indeed, different from standard scientific methods advocated not only in the nineteenth century but today as well, including hypothetico- deductivism, inductivism, and inference to the best explanation (IBE). It is a method that many scientists (whether knowingly or not) have employed in developing and defending a theory they could not prove. Maxwell's position is not that following the method will necessarily yield truth, or justified belief, or even a theory worth considering, but that it can do so if the development and defense are sufficiently good, and that it is a reasonable and useful strategy to follow when experimental proof is not available. I propose to formulate and illustrate the method, see how it differs from the others noted previously, and explore and defend its virtues. I begin with a characterization of the contrasting methods.

1.