“Isn’t evolution JUST a theory, anyway?”
This confusion is a prominent one. It stems from the multiple definitions of the word “theory.” In colloquial language, the word “theory” means an idea or a guess; but in science, a theory is a well-established, well-supported explanation of a collection of observations. It ties together all the observed facts and can be used to make predictions about future experiments.
People often think there is an increasing ladder, or hierarchy, of confidence for scientific explanations: first comes the hypothesis, then after some testing it becomes a theory, then it becomes a law, and once it is confirmed it becomes a fact. But this is simply not the case. Facts are just repeated observations. It is a fact that when you drop a ball on earth it accelerates downwards at roughly 9.8 m/s2, but that if you did the same thing on the moon it would accelerate at 1.6m/s2. It is the theory of gravity that explains why the balls accelerate at these speeds, along with many, many other observations. This theory allows us to make predictions for future experiments. As the french polymath Henri Poincaré wrote (in Science and Hypothesis): “Science is constructed of facts as a house is of stones. But science is no more a collection of facts than a house is a heap of stones.” Facts are essential, but they would have little meaning were they not held together by scientific theory. Theory is the logical structure that ties together an otherwise seemingly random array of observation and allows for the possibility of making predictions. This is precisely what allows us to design the crucial experiments that expand our current knowledge and push the scientific frontier.
Creationists, intentionally or not, mislead students as to the meaning of the words “scientific theory” and attempt to diminish the status of evolution. Students are told that a theory is merely a hypothesis that has been tested successfully numerous times, but what theories are really striving to become are laws. Laws, they are told, are scientific facts because they have been scientifically proven. After establishing these definitions students are led to believe that evolution is only a theory – and not a law or a fact – discounting the intellectual achievement that it represents. Furthermore, because a theory isn’t “proven,” opposing explanations should also be presented, such as young-earth creationism and intelligent design. While the arguments sounds compelling, they are misguided. The scientific definition of law is a descriptive generalization of a phenomenon. No matter how extensively a theory is tested and despite incontrovertible data supporting its accuracy, a theory cannot become a law. They are categorically different; Laws describe things, theories explain them. In fact, if there were a hierarchy, theories would actually be above laws. There is nothing higher, or better, than a theory. In this light, arguing that evolution is JUST a theory really doesn’t make any sense. After all, if evolution is JUST a theory, then gravity is also JUST a theory. The earth revolving around the sun is JUST a theory. The germ theory of disease is JUST a theory.
The reason scientists are hesitant to use the word “proven” as opposed to “a well-established and well-supported explanation that conforms with all known knowledge” is not because they question the validity of the explanation but because they recognize the possibility of future findings. As Harvard paleontologist Stephen Jay Gould put it: “In science, fact can only mean confirmed to such a degree that it would be perverse to withhold provisional assent. I suppose that apples might start to rise tomorrow, but the possibility does not merit equal time in physics classrooms.” Despite the fact that apples have fallen to the ground yesterday, today, and probably will tomorrow, no one seems to be passing legislation to teach opposing positions to the theory of gravitation.
So to sum up: Evolution is not JUST a theory, it is TRIUMPHANTLY a theory!