James Q. Wilson explains it as clearly as is possible:
The other meaning of theory is the popular and not the scientific one. People use “theory” when they mean a guess, a faith or an idea. A theory in this sense does not state a testable relationship between two or more things. It is a belief that may be true, but its truth cannot be tested by scientific inquiry. One such theory is that God exists and intervenes in human life in ways that affect the outcome of human life. God may well exist, and He may well help people overcome problems or even (if we believe certain athletes) determine the outcome of a game. But that theory cannot be tested. There is no way anyone has found that we can prove empirically that God exists or that His action has affected some human life. If such a test could be found, the scientist who executed it would overnight become a hero.
Evolution is a theory in the scientific sense. It has been tested repeatedly by examining the remains of now-extinct creatures to see how one species has emerged to replace another. Even today we can see some kinds of evolution at work, as when scholars watch how birds on the Galapagos Islands adapt their beak size from generation to generation to the food supplies they encounter.
***Proponents of intelligent design respond by saying that there are some things in the natural world that are so complex that they could not have been created by “accident.” They often use the mousetrap as a simile. We can have all of the parts of a trap–a board, a spring, a clamp–but it will not be a mousetrap unless someone assembles it. The assembler is the “intelligent designer.”
***What schools should do is teach evolution emphasizing both its successes and its still unexplained limitations. Evolution, like almost every scientific theory, has some problems. But they are not the kinds of problems that can be solved by assuming that an intelligent designer (whom ID advocates will tell you privately is God) created life. There is not a shred of evidence to support this theory, one that has been around since the critics of Darwin began writing in the 19th century.
Some people worry that if evolution is a useful (and, so far, correct) theory, we should still see it at work all around us. We don’t. But we can see it if we take a long enough time frame. Mankind has been on this earth for about 100,000 years. In that time there have been changes in how people appear, but they have occurred very slowly. After all, 1,000 centuries is just a blink in geological time.
Of course, those observations are pointless when dealing with the ‘young earth’ crowd, and all ID proponents are doing is providing creationists with cover. Thank goodness, people are beginning to see through it.
As a side note, did any of you have to read James Q. Wilson’s texts as an undergrad?