Scientific theories can be used to make predictions. Some of these predictions are successful in the sense that they turn out to be true. And sometimes a theory successfully predicts the existence of a phenomenon that was previously unknown. How could this happen? One seemingly plausible explanation is that the theory in question must be true. After all, how could a false theory make this sort of successful prediction? But there are, in fact, examples of false theories that have successfully predicted previously unknown phenomena.
One such example concerns the discovery of the chemical element boron. Three chemists are credited with its discovery: the English chemist Sir Humphry Davy (1778–1829) and the French chemists Joseph Louis Gay-Lussac (1778–1850) and Louis Jacques Thénard (1777–1857), who worked together but independently of Davy. In 1808, these chemists were the first to decompose the substance that chemists at the time referred to as boracic acid (what we would call boron trioxide, B2O3, which is the anhydride of what we would call boracic acid). They did so by using potassium to extract the oxygen from boracic acid and thereby succeeded in isolating samples of boron.
Of course, before they succeeded in decomposing boracic acid, these chemists didn’t know for sure that it contained oxygen. So why did they suspect that boracic acid contains oxygen?
The answer to this question involves the French chemist Antoine-Laurent de Lavoisier (1743–1794). Today, Lavoisier is known primarily as the chemist who discovered the role that oxygen plays in combustion. What most people today don’t know is that Lavoisier assigned another role to oxygen, namely, that of the acidifying principle, i.e., the substance that gives acids their acidic properties. Lavoisier arguably felt that this role of oxygen was even more important than its role in combustion. After all, the term that Lavoisier proposed for this particular substance, namely, ‘oxygen’ (or, more precisely, the French term ‘oxygène’), is derived from the Greek words for ‘acid generator.’ According to Lavoisier’s oxygen theory of acidity, all acids contain oxygen (the acidifying principle) and something else, which Lavoisier called the base or radical of the acid. One of the main ways in which acids differ from each other concerns their radicals. For example, sulfuric acid and phosphoric acid are different acids because they have different radicals, namely, sulfur and phosphorus. But the fact that these substances are both acids, as opposed to something else, is due in part to the fact that they both contain oxygen.
So why did chemists at the time suspect that boracic acid, which no chemist had yet succeeded in decomposing, contains oxygen? For Gay-Lussac and Thénard, the answer was that they were committed to Lavoisier’s theory of acidity. Since boracic acid is an acid, Lavoisier’s theory predicted that it must contain oxygen. Lavoisier’s theory therefore provided Gay-Lussac and Thénard with a reason for trying to decompose boracic acid by using a substance like potassium, which has a great affinity for oxygen. This method worked, and the prediction that boracic acid contains oxygen was successful. In this sense, Lavoisier’s theory led Gay-Lussac and Thénard to their discovery of boron.
But in that case, a false theory led to the discovery of boron. The idea that oxygen is the acidifying principle, i.e., the substance that gives acids their acidic properties, was rejected long ago. And today, we know of plenty of acids that contain no oxygen. In fact, chemists at the time knew of one such example. In 1787, the French chemist Claude Louis Berthollet (1748–1822) had already shown that prussic acid (hydrocyanic acid, HCN) contains no oxygen. And just a couple of years after the discovery of boron, Davy provided some compelling reasons for concluding that muriatic acid (hydrochloric acid, HCl) doesn’t contain any oxygen either.
Even though Lavoisier’s theory is false, it successfully predicted that boracic acid contains oxygen. How could a theory that is just plain false lead to a prediction that turns out to be true? Philosophers of science who have engaged with this question generally fall into one of two camps. There are those who think that, if we just dig a bit deeper, we’ll find some kernel of truth in that false theory that explains how it could make a true prediction. And there are those who think that we should abandon the attempt to explain why a false theory makes true predictions in terms of the idea that the theory must contain even a kernel of truth. When it comes to Lavoisier’s oxygen theory of acidity, I’ve done some digging myself, and I’ve been unable to find that kernel of truth that would explain why he got it right about boracic acid. After all, when it came to other acids (for example, muriatic acid), he got it quite wrong. Perhaps we just need to learn to be more comfortable with the fact that theories sometimes get things right for the wrong reasons.
Based in part on Hricko, J. (2021). What Can the Discovery of Boron Tell Us About the Scientific Realism Debate? In T. D. Lyons & P. Vickers (Eds.), Contemporary Scientific Realism: The Challenge from the History of Science (pp. 33–55). New York: Oxford University Press. https://doi.org/10.1093/oso/9780190946814.003.0003
Jonathon Hricko is Associate Professor at the Institute of Philosophy of Mind and Cognition of National Yang Ming Chiao Tung University.