Theories have limits. Some account for phenomena quite well when applied within given parameters, only to fail miserably when expanded beyond them. Ironically, it is often the more limited theory that appeals most strongly to common sense. Consider the case of Ptolemy.

Ptolemy (ta-le-mi) was a 2nd-century astronomer who, like virtually all of his contemporaries, believed that the heavens revolved around the Earth. This geocentric conception of the universe worked quite well for the ordinary living of his day. In fact, it still does.

When I leave for work in the morning, the sun is rising in front of my house. By the time I get home, it's setting in the backyard. The house is just where I left it, so common sense argues that the sun is moving and the house is stationary. This notion, although misguided, is adequate to the demands of routine commerce.

As an astronomer, however, Ptolemy had a problem. Trying to track the paths of the known planets of his time, he noticed that they sometimes appeared to have moved in the wrong direction from one night to the next, only to later resume their normal rotation.

Because he "knew" that the planets were revolving around the Earth, he developed the idea of "epicycles" to explain the anomalies. The hypothesized epicycles were little loops within the planetary orbits -- in effect, the planets of his theory occasionally executed pirouettes while traversing the sky.

This "dancing planet" explanation was considered to be settled science for more than 14 centuries until Copernicus came along with the radical notion that the Earth was merely one of the planets and like its fellows, it revolved around the sun.

Copernicus' heliocentric theory was not an immediate hit. It defied common sense; and initially, Ptolemy's formulation better predicted the future positions of the planets. Only after Kepler realized that planetary orbits were elliptical -- rather than circular as both Ptolemy and Copernicus assumed them to be -- was that problem resolved and our modern conception of the solar system began to gain acceptance.

Galileo, incidentally, spent the last decade of his life under house arrest as a convicted heretic for trying to advance Copernican apostasy in Italy where the ruling Roman Catholic Church accepted geocentrism as an article of faith.

The persecution of Galileo is often cited as a prime example of the conflict between dogma and science. It is generally regarded as a cautionary tale about the hazards of subjugating intellectual inquiry to religious doctrine, positing a compelling argument in favor of academic freedom within a civil society.

To dismiss the episode, however, as yet another good reason for the separation of church and state is to miss the larger point: namely, that everybody involved in the history of the dispute -- from Ptolemy to the pope -- was to some degree blinded by his own perspective.

Much to his credit, Ptolemy didn't simply ignore empirical observations that didn't fit his preconceived ideas. Though he was trapped within the limitations of his mistaken assumption about the Earth's place in the cosmos, he still modified theory to accommodate data -- a legitimate exercise of scientific reasoning. He would eventually be proved wrong, but it would take a while.

Imagine how difficult it must have been to accept Copernicus' idea. While the learned classes had understood from the time of Pythagoras that the Earth was spherical, no one had ever seriously suggested that it was not the center of the universe. Not only did geocentrism conform to observations of everyday reality, but it was both functional and flattering.

Men were able to traverse the globe by land and sea without challenging the doctrine. (Space travel would have been problematic, but NASA was chronically underfunded during most of the 16th century.) To make matters worse, because of Copernicus' unquestioned assumption that planetary orbits were circular, the old theory was better at prediction than the new one.

Further, the geocentric viewpoint seemed to fit neatly within the message of Genesis that mankind was the centerpiece of God's creation. Copernicus couldn't be right because his theory conflicted with a higher truth: If we were the Almighty's crowning achievement, what were we doing living out here in the celestial boondocks? Understandably, clerics felt their beliefs threatened by such secular philosophic speculation.

Of course, Copernicus' was not the final answer to the riddle of creation. Newton would add his theory of gravitation to the Copernican model to explain the mechanics of the solar system. That basic paradigm would endure until Einstein extended his investigations to cosmic dimensions, thus upsetting the apple cart with his general and special theories of relativity.

Upon learning of Einstein's work, George Bernard Shaw remarked,

"Ptolemy created a universe that lasted a thousand years.

Copernicus created a universe that lasted four hundred years.

Einstein has created a universe, and I can't tell you how long it will last."

The point here is that science is never settled and consensus is often wrong. So long as human knowledge is incomplete, all theories are vulnerable to falsification because science is a method of investigation that subjugates belief to continued empirical verification.

The dogmatic mind begins with truth then looks for evidence to substantiate what it already knows. By contrast, the scientific intellect seeks evidence to learn the truth. In either instance, it is often the unspoken assumptions that ultimately speak the loudest.