Hello! My question concerns the word, "theory". Can a theory be considered fact, and what gives one theory more credibility than another? I know that some theories are empirical, and can be tested scientifically for validity. So if a theory such as evolution seems a fact, why is it still called a theory? Should it not be fact? Obviously, some non-empirical theories, like String Theory, can't as yet be tested, and are questionable. But scientifically, do empirical theories get closer to truth, and can some be called true?

The word "theory" has a common meaning, which is something like "hypothesis" or "speculation." It also has a scientific meaning, which, close enough for our purposes, is "organized set of principles." When we call something a theory in that sense, we aren't saying anything at all about whether the principles are true or false.

Keep in mind that the word "theory" even gets used in mathematics---for example, when mathematicians talk about number theory (roughly, the study of the properties of whole numbers.) The word "theory" here isn't meant to suggest that the principles number theorists use are suspect.

The "theory/fact" confusion is unfortunate. Evolutionary theory is a theory in the scientist's sense: an organized collection of explanatory principles. As it turn out, those principles have been very successful tools for making sense of nature.

So why not just call these principles facts? We could, but theoretical principles tend to be abstract and general. We tend to use the word "fact" for the sorts of things that are closer to the ground---the kinds of things that people who disagree about abstract principles might see as evidence one way or another. But even if the theory's principles are beyond serious doubt, scientists will still call it a theory because given what they mean, that's what it is.

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