7. The Need for Negativity

It may sound strange to praise negativity. After all, we are told again and again not to be negative, in a culture which has embraced "positive thinking" and "self-esteem" as prerequisites for a healthy society. But like it or not, there is a time and a place for negativity, particularly in scientific theories, which must say not only what will happen, but also what will not happen.

Why is it so important for a scientific theory to say what won't happen? Consider this: why is the equation 2+2=4 useful? Not just because it says that 2+2 is equal to 4, but because it also says that 2+2 is not equal to 3, or 5. If mathematical equations were not so specific, they would be useless. Why is the statement "The suspect is a white male, brown hair, aged 25 to 35, six feet tall" useful? Because it allows you to "narrow down" your search, by not only telling you what he is, but also telling you what he is not. He is not female, he is not 5 feet tall, he is not 60 years old, etc. Even if your picture isn't perfect, you've at least got a much better idea than "could be anything".

Science functions along similar lines. Newton's third law of motion would be quite useless if its prediction of an equal and opposite reaction did not have the corollary that the reaction force will not be only half the action force, and it will not be at right-angles to the action force rather than being directly opposite to it. Imagine a physics version of creationism, where it is assumed that God decides what reaction forces will be. This alternative theory would "predict" the behaviour of reaction forces only by copying the predictions of Newton's third law because it has no real predictive ability of its own (just try calculating a force vector from "God did it"), and it would declare itself a default winner if anything ever happened which should seem to contradict that law. One could defend it in much the same way creationists defend their ideas: by simply declaring that you can't prove it's not true. But we can show that it is useless.

Of course, the obvious question is: does evolution theory do the same thing? Does evolution theory make only vague, non-specific predictions? Does evolution theory fail to predict what can not happen, thus making itself unfalsifiable and untestable? Contrary to creationist accusations, it does not. In fact, it says there are a number of characteristics which should never occur in any species on Earth. Without the fanciful construct of a time machine, we can test these criteria today, by looking at existing animals and the fossil record.

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