Theory and Fact

     "My son is taking piano lessons, and I'm pleased with that. I just wish they wouldn't teach music theory as if it were fact."

     There's a widespread misconception about what academics and scientists mean by the word "theory". In common usage, we often speak of "theory" as if it were a guess, a tentative fact-in-waiting yet to be confirmed, and so we'll often hear a complaint like the above with respect to evolution: people object to it being taught as if it's fact when it's "only a theory".

     Of course, that's not what "theory" means at all, and thinking about music theory is a good way to understand that. As you probably know, music theory includes all those complicated symbols composers and arrangers use to write down musical scores, which was the only way to preserve a record of compositions back in the days before audio recording. But there is more to musical theory than simply notes, rests, time and key signatures. Musical theory is a systematic way of describing music, relying on a set of defined terms and symbols and rules relating those terms to one another in various ways. With musical theory one can not only write down a melody so someone else can play it, but analyze and manipulate musical propositions, and even make predictions such as which notes will make for pleasant harmonies with which others.
     In short, musical theory is a cognitive framework for understanding music. The key word here is "understanding", not "knowing". Musical theory is not something you can know in the sense of justified true belief; it simply isn't a matter of being true or false. Rather, it's a matter of being more or less helpful in making sense of music.
     So it is with other theories: game theory, number theory, relativity theory and yes, evolutionary theory. A good theory enhances our ability to make meaningful and accurate inferences or predictions about the subject matter. These inferences or predictions themselves are factual claims, and a theory lives or dies on the accuracy of these predictions; if a theory leads to inferences that turn out to be false, well, the usefulness of the theory is reduced. Useless theories aren't false; they're just not used.
     Mind you, a theory that always makes true predictions isn't necessarily useful; it may just be ridiculously vague. Creationism (which explains all observed phenomena with "Because God made it that way") always gives a "true" prediction to the question "What will happen if we do this experiment?"  But "Whatever God makes happen" is a very uninformative answer, even if it is true (which, if an omnipotent God exists, it must be by definition). This is why scientists talk about good theories making falsifiable predictions.

     So to complain that evolution is "just a theory" and therefore shouldn't be taught as "fact" is simply to misunderstand what a theory is. Of course, that doesn't really answer the objection; the creationist who uses the word "theory" correctly will complain instead that the predictions of evolutionary theory, which often are presented as facts (and justifiably so), are false. And that's fine, because then we can have an intelligent conversation about what the facts are, with reference to evidence and principles of reasoning and so forth, and such a conversation does properly answer the objection.