James Q. Wilson has a terrific op-ed piece about evolution, ID and the nature of science in the Wall Street Journal. Perhaps the most important part of the column is a section about the meaning of the word “theory”. This is by no means a revolutionary passage, but the colloquial meaning of the word is so pervasive that it cannot be repeated often enough:
Some people will disagree with his view, arguing that evolution is a “theory” and intelligent design is a “theory,” so students should look at both theories.
But this view confuses the meaning of the word “theory.” In science, a theory states a relationship between two or more things (scientists like to call them “variables”) that can be tested by factual observations. We have a “theory of gravity” that predicts the speed at which two objects will fall toward one another, the path on which a satellite must travel if it is to maintain a constant distance from the earth, and the position that a moon will keep with respect to its associated planet.
This theory has been tested rigorously, so much so that we can now launch a satellite and know exactly where it must be in space in order to keep it rotating around the earth. It was not always that way. From classical times to the Middle Ages, many important thinkers thought that the speed with which an object falls toward the earth will depend on its weight. We now know that this view is false. In a vacuum, the two objects will fall at the same speed and, thanks to Newton, we know the formula with which to calculate that speed.
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.