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Writing in the Washington Post on Thursday, May 5, 2005, Peter Selvin said: "Alarmed by proposals to change how evolution is taught, scientists and teachers are mobilizing to fight back, asserting that educational standards are being threatened by what they consider a stealth campaign to return creationism to public schools." He explained to his readers that "intelligent design [creationism][is] a carefully marketed theory that challenges accepted understandings of Earth's origins in favor of the idea that a creator played a guiding role."

What is a theory? I recently asked this question of a relatively learned friend. He answered by comparing theory to fact, stating that a theory was a non-substantive explanation for a set of facts. As more facts become known, he said, theoretical explanations give way to factual ones. Ultimately, a theory may be vindicated - proved by fact; or partially vindicated - partially substantiated by fact, but also partially disproved; or overturned.

In one sense my friend is right. When a historian uses this word, one can assume he gives it my friend's meaning. And when a poet uses the word. And a writer. Almost any non-scientist, in fact. On the other hand, when a scientist or engineer means what my friend defined, he uses the word "hypothesis" or "hypothetical." When he uses the word "theory" or "theoretical," however, he means something entirely different.

In scientific usage "theoretical" is the opposite of "empirical." In measuring the speed of sound in sea water, for example, researchers frequently use an equation that consists of a long series of increasingly smaller functions of density, salinity, temperature, and pressure. This equation is the result of measuring the speed of sound in sea water under a very large number of differing situations, and then deriving the equation from these data. It is called an empirical equation.

Another approach to the same problem is to create a mathematical model of the ocean, and to derive an equation for the speed of sound that depends upon the mathematical structure of this model. This equation is called a theoretical equation.

Both equations are real. One is derived empirically, the other theoretically. Each is subject to error, and each is only as good as its ability to predict the actual speed of sound in any given situation. Ultimately, scientists attempt to replace empirical equations with theoretical ones, as they gain a deeper understanding.

Albert Einstein's Theory of Relatively is

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