What is a theory?

I've been meaning to blog on this very topic for a long time, but, via Pharyngula, I've found an article by Steve Olson published in The Washington Post that explains this point better than I probably could do myself. One of the most frequent canards that creationists use is that evolution is "just a theory." In fact, a lot of those idiotic warning stickers required on biology textbooks by various school districts say this very thing. The problem is, the word "theory" has a very different meaning in science from its meaning in colloquial use. But just let Olson explain:

If you want to know one reason why the debate over teaching evolution remains so contentious, consider the stickers some school boards have wanted to paste in high school biology textbooks. They label evolution a "theory, not a fact," suggesting that an alternative explanation is possible.

It's a clever strategy. Even people sympathetic to evolution often don't know how to respond to the assertion that evolution is "just a theory." And the opposite claim -- that evolution is a fact -- can generate skepticism among those who don't like to be told what to think.

But these stickers use the words "theory" and "fact" in a very misleading way. The biggest problem is that "theory" has two separate meanings. In common usage, "theory" means an idea or a hunch: "I have a theory about why she left him." No one really knows what the reasons were, but we can guess.

That's not what "theory" means within science. When scientists speak of the theory of gravitation, cell theory or evolutionary theory, they are talking about scientific concepts that have been so thoroughly tested that they are very unlikely to change. Theories are the results of decades or centuries of scientific effort. They draw on many interconnected observations and ideas. They are the end products of science, not stages on the way to the truth.

In science, a hunch or conjecture is called a hypothesis, not a theory. When Copernicus proposed in the early 16th century that the Earth revolves around the sun rather than vice versa, his idea was a hypothesis. But four centuries of observation and thinking have convinced us that heliocentrism is a theory, not just an intriguing idea. It is compatible with everything we know about the solar system and explains observations that cannot be explained in other ways.

Ideally, English would have a different word for these comprehensive organizing concepts in science. But for now, "theory" is doing double duty. So calling evolution a theory may seem to denigrate it in everyday terms, but in scientific terms that's high praise.

Precisely. The Theory of Evolution is just as rich and based in evidence as the Theory of Relativity or any of the other major theories in science, and the reason scientists won't take creationists seriously is because they challenge a well-founded and accepted theory without presenting strong evidence contradicting evolution and/or supporting their ideas. (Alties frequently do the same thing.) My only quibble with the article is that Olson doesn't really explain how old theories are usually incorporated into new theories at some level, mainly because theories by scientific definition represent the best explanations for a scientific phenomenon that presently exists. (The contrarian in me wants to use this simple observation to illustrate that the advancement of scientific knowledge is usually evolutionary, not revolutionary.) For example, the theories behind Newton's Laws of Motion were not disproven by the Theory of Relativity. Instead, Einstein showed that Newton's Laws were a special case of relativity for situations in which the velocity is much less than the speed of light, so that the factor v/c approaches zero and Einstein's equations reduce to Newton's Laws. Given that such slow velocities were all that Newton could observe in his time, he was absolutely correct to derive the laws that he did.

I agree with PZ. We should never, ever refer to creationism or intelligent design as "theories." They are not. They don't even rise to the level of hypotheses. It's also important to remember that science is not logic. Logic is an important part of the scientific method but does not necessarily have much to do with how natural phenomena operate. Many are the scientists who have been misled by an idea of how they thought nature should be, based on logic, as opposed to how nature actually IS, based on evidence and experimentation. (After all, Lamarckian evolution is a wonderfully logical concept--logical and wrong, not the way nature operates.) "Intelligent design" creationists frequently use logic (usually bad logic, but logic nonetheless) to justify their "hypotheses." However, logic alone is not enough. Science is about evidence, experimentation, hypotheses, and reasoning. They all have to be consistent with each other for a principle to be considered valid. Creationism and its deceptive offspring intelligent design fail on all these counts.