The validity of one the most (if not the most) beautiful and groundbreaking scientific discoveries of all time – Darwinism – is often disputed with a common argument that it is, after all, just a theory. This not-so-rare refrain, especially popular among Creationists, is based on a flawed notion, a misunderstanding of what a scientific theory actually is.
Here’s a definition from Wikipedia: “A scientific theory is a well supported body of interconnected statements that explains observations and can be used to make testable predictions.”
There are two interesting points to make here:
(1) In science, a theory is much more than a speculation, much more than a mere hypothesis. (This might seem trivial to some, but the creationists don’t seem to get this and often dismiss the theory of evolution based on that convenient misunderstanding.)
In a general sense, a theory is considered to be more like a ‘point of view’ or a hypothesis, far away or off from a fact. But in science, a theory has a much stricter connotation. A scientific theory is much closer to fact – often as close as we can get to the fact.
As Jerry Coyne explains in Why Evolution Is True — for a theory to be considered scientific, it must be testable and make verifiable predictions. A theory is considered “true” when it is proved repeatedly, when we have accumulated enough evidences in its support. There’s no tipping point, after which a scientific theory becomes a fact.
And that brings me to the second point:
(2) Taking another quote from WEIT: All scientific truth is provisional, subject to modification in light of new evidence. There’s no alarm bell that goes off to tell scientists that they’ve finally hit the ultimate, unchangeable truths about nature. Scientists, unlike zealots, can’t afford to become arrogant about what they accept as true.
In a brilliant post (that blew my mind away) Sean Carroll writes about a very wrong way of thinking about science: “the idea that true and reliable knowledge derives from rigorous proof, and anything less than that is dangerously uncertain.” Referring to an example of Einstein’s theory of General Relatively, Correl writes:
Even when we do believe [a scientific] conclusion beyond any reasonable doubt, we still understand that it’s an approximation, likely (or certain) to break down somewhere. There could very well be some very weakly-coupled field that we haven’t yet detected, that acts to slightly alter the true behavior of gravity from what Einstein predicted. And there is certainly something going on when we get down to quantum scales; nobody believes that GR is really the final word on gravity. But none of that changes the essential truth that GR is “right” in a certain well-defined regime. When we do hit upon an even better understanding, the current one will be understood as a limiting case of the more comprehensive picture.
Scientific theories are never proven completely. They are supported by facts, but they never become (ultimate, unchangeable) facts. As Carroll writes, we have to take “I believe x” to mean, not “I can prove x is the case”, but “it would be unreasonable to doubt x.” The absence of a reasonable doubt (based on evidences) is what makes a scientific theory “true”. But the doubt is never cleared completely – we can’t be absolutely sure about anything.
Also see Evolution as Theory and Fact entry on Wikipedia.