In Miller’s Crossing (1990), one of my favorite movies of all time, the corrupt chief of police O’Doole speculates that the boss of the town, Leo O’Bannon, is losing his grip, whereupon O’Bannon’s right-hand man Tom Reagan reproaches him, ending, “there are plenty of coppers I know who wouldn’t mind bein’ chief and could swallow it clean.” Hastily backpedaling, O’Doole protests, “Jesus, Tom, I was just speculatin’ about a hypothesis.” Here, however, I’m going to speculate not about a hypothesis but about the concept of a hypothesis.
The occasion is my colleague Stephanie Keep’s recent post “Hypotheses, Theories, and Laws, Oh My!” Keep suggested, “Hypotheses and theories are explanations for phenomena that differ in breadth, not necessarily in degree of evidentiary support,” adding, “A hypothesis is an explanation for a relatively narrow set of phenomena and a theory is an explanation for a relatively wide set of phenomena.” The suggestion was in the service of debunking a misconception: “With enough evidence, a hypothesis can become a theory, which can become a law.”
On that definition of “hypothesis,” it’s clear that hypotheses aren’t promoted to theories by the addition of evidence (although perhaps they could be by the expansion of scope). But that definition isn’t obligatory, of course. Buried somewhere among the 180 comments to Keep’s post, Gregory Mead expresses his acceptance instead of the definitions of “hypothesis,” “theory,” and “law” offered in the National Academy of Sciences publication Teaching about Evolution and the Nature of Science (1998):
Law: A descriptive generalization about how some aspect of the natural world behaves under stated circumstances.
Hypothesis: A testable statement about the natural world that can be used to build more complex inferences and explanations.
Theory: In science, a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses.
Mead suggests that the National Academy of Sciences definitions have as a consequence that “a hypothesis is basically an untested explanation, and CAN turn into a theory if it’s tested thoroughly, and accepted by scientists” (emphasis in original). Keep wasn’t willing to dig in her heels, responding, “I think you’ll find that these are not universal definitions. So, I think setting operational definitions is fine, and can even be helpful!…it’s important to acknowledge that everyone is not reading from the same playbook.”
I want to dig in my heels a little, though, because I think that Keep is right that it’s a mistake to suppose that hypotheses are promoted to theories by the addition of evidence. But I think that both her definition and the National Academy of Sciences definition of “hypothesis” are misleading. On a definition of “hypothesis” that recognizes the distinctive role played by a hypothesis in science, it becomes clear why hypotheses are not promoted to theories by the addition of evidence, why the misconception that Keep was targeting is a misconception.
Philosophers of science tend not to offer definitions of terms as basic as “hypothesis,” but here’s a useful definition from John Stuart Mill’s System of Logic (1843): “An hypothesis is any supposition which we make (either without actual evidence, or on evidence avowedly insufficient) in order to endeavor to deduce from it conclusions in accordance with facts which are known to be real; under the idea that if the conclusions to which the hypothesis leads are known truths, the hypothesis itself either must be, or at least is likely to be, true.”
Victorian syntax aside, the basic thought is clear: a hypothesis is a statement that is entertained tentatively or provisionally, by way of supposition or conjecture, for the sake of further investigation. Mill’s definition is not restricted to science—he thinks that there are hypotheses in mathematics (although he famously thinks that mathematics is a branch of science). In science, then, a hypothesis is a testable statement about the natural world that is advanced tentatively or provisionally, by way of supposition or conjecture, for the sake of further investigation.
(The Oxford English Dictionary, with comparably Victorian syntax, agrees: a hypothesis, in the relevant sense given, is defined as: “in the sciences, a provisional supposition from which to draw conclusions that shall be in accordance with known facts, and which serves as a starting-point for further investigation by which it may be proved or disproved and the true theory arrived at.” Illustrative quotes include passages from Thomas Browne, Robert Boyle, and Thomas Henry Huxley as well as from Mill, which is a fairly impressive pedigree.)
What’s present in the Millian definition of “hypothesis”—and absent from Keep’s and the National Academy of Sciences definitions—is the distinctive use of hypotheses in science. The difference between hypotheses and theories, on the Millian definition, is not primarily a matter of what they say but a matter of what scientists do with them. Accordingly, on the Millian definition, it’s helpful to think of the activity, hypothesizing or theorizing, as primary, and of the statement that’s the object of the activity, the hypothesis or the theory, as secondary.
To make the point concrete, consider two possible assignments for a science class. In the first, students are given a list of statements, each of which they are expected to classify as hypothesis or theory (for example). In the second, students are given a text with a narrative of a scientific investigation, and expected to identify which passages describe hypothesizing and which passages describe theorizing. Which assignment will induce a greater understanding of the nature of science? If you prefer the second, as I do, then you see the appeal of the Millian definition.
Applying the Millian definition to the question at hand, it seems fair to say that although it is occasionally possible for a statement to be useful for both hypothesizing and (albeit not at the same time) theorizing, there’s no reason to expect that always to be the case. Statements used in hypothesizing are often, as Keep emphasizes, narrow in scope, which facilitates their testing but inhibits their usefulness in theorizing. And in general, a hypothesis with a lot of evidence in its favor becomes not a theory, but, if anything, a fact confirmed by inference.