I've lost count of how many times I've heard or seen fundamentalist theists (whether Christian or Muslim) disparage the entire concept of evolution by saying, "it's just a theory." As in, "Scientists claim that man evolved from apes, but the Theory of Evolution is just that -- a theory! It's nothing more than a guess!" I've also lost count of how many times I have heard or seen people (whether atheists or just rational theists) respond to this claim, but the responses always seem to be one of two different approaches. Some people go with a glib response to the tune of, "Evolution is 'just' a theory the same way gravity is 'just' a theory!" Others point out that the word "theory" has a different meaning when used in a scientific context than it does when used colloquially. In other words, while theory can certainly mean "simply a guess or conjecture" when used colloquially, when scientists use the term they mean "a coherent group of tested general propositions, commonly regarded as
correct, that can be used as principles of explanation and prediction
for a class of phenomena."
Neither of these two standard responses are particularly helpful, in my opinion. The first suffers simply because it is glib, and doesn't really offer enough information to change anybody's mind on the subject. Glib responses, in my experience, are best suited to making the person making the response feel superior, but don't typically have much affect on the respondent. The second response, while informative and accurate, suffers because it completely misses the entire point. It doesn't really matter if "theory" is defined to mean that it's not "just" a guess but is instead supported by evidence and generally accepted as true. That still lets fundamentalists claim that it doesn't have to be true. "After all," they might argue, "for centuries it was generally accepted by scientists that the earth was flat, or that the sun revolved around the earth, or that everything was made up of earth, air, fire and water."
No, I think the best response to the whole "it's just a theory" argument is to point out what the Theory of Evolution actually is, not what the word "theory" means. And no, I don't mean explain all the details of the theory and point out all the evidence that supports it (although that can certainly be helpful if you have the scientific background to pull it off). I'm talking about something a lot more basic which always seems to get missed in these discussions. It is important to explain that the Theory of Evolution is not the proposition that there is such a thing as evolution in the first place, that all currently existing species (including man) have evolved from previously existing species, and that all life on earth shares a common ancestor who lived billions of years ago. Instead, the Theory of Evolution is the proposition to explain how and why all of that took place.
Evolution, in other words, is an observable, demonstrable fact and not a theory at all! The Theory of Evolution is our best explanation (supported by evidence and commonly accepted as accurate) as to what caused (and still causes) that fact. And just because our best explanation might be incomplete or inaccurate or just flat-out wrong doesn't say anything about whether scientists are at all unsure as to whether evolution is a real thing. This is similar to how the "Theory of Gravity" does not seek to explain whether or not there is gravity, but instead seeks to explain why there is gravity and how it works.
Evolution is an observable and demonstrable fact, plain and simple. We have a multitude of evidence from various sources, such as the fossil record, comparative anatomy, DNA analysis, etc., that shows unequivocally that all life on earth has evolved from prior life forms over time and that all living creatures shared common ancestors in the past. Evolution itself is not a theory -- it's simply an observation. The Theory or Evolution deals with how and why evolution occurred, and the commonly accepted explanation is that evolution is caused by the occurrence of random mutations within a population that gives rise to variety, and that changes in environment cause different variations within the population to either thrive or perish, which over vast time scales can lead to entirely new species, genera, orders, classes, phyla and even kingdoms.
Whether this theory is wholly accurate and complete can certainly be discussed. It is, after all, "just" a coherent group of tested general propositions, commonly regarded as correct, that can be used as principles of explanation and prediction. Maybe there are additional factors at work that we haven't figured out yet. Maybe some of the factors we currently believe to be involved aren't as important as we think. Maybe we've got it completely wrong and there is a totally different explanation for how evolution has occurred (and is still occurring). Maybe that explanation is even "God did it" (or "aliens did it" or "magic pixies did it"). But none of that uncertainty changes the fact that evolution has occurred and continues to occur.
Evolution is a fact. The explanation as to how it works is a theory. A very good, commonly accepted theory that can be and has been used as principles of explanation and prediction, but a theory nonetheless. And this, I believe, is the best response to the whole "evolution is just a theory" argument. No, the "Theory of Evolution" is a theory, but evolution itself is an an accepted, observable, demonstrable fact.
