For centuries, religious believers have sought signs of God's designing hand in nature's mysteries - whether the orderly motions of the sun, moon and planets, the intricate beauty of an insect's wing, or the complexity of the human eye.

Others say it's the nature of faith not to require evidence.

The term intelligent design entered popular discourse recently, but the philosophy behind it goes back to antiquity. Some philosophers, theologians and religious scientists say the age-old battle unnecessarily pits religion against science, and religion tends to lose.

University of Pennsylvania anatomy professor Peter Dodson said his Catholic faith does not require an intelligent designer to leave clues in nature. "I'm an evolutionary biologist and I'm a Christian, and these issues are not problematic for me."

Dodson, who has lectured on the relationship between religion and science, said the intelligent-design argument falls into an old philosophical notion called "God of the gaps" - the search for signs of the supernatural in otherwise unexplainable natural phenomena.

Isaac Newton proposed something like this, said Wesley Wildman, a professor of theology at Boston University. Where he failed to explain the more complex interactions between heavenly bodies with his own laws, Newton proposed the hand of God intervening directly to keep the solar system orderly.

Today's proponents of intelligent design have argued that science can't explain the complexity of some single-celled organisms or the machinery of the human eye. They suggest a designer's intervention.

Filling the gaps

The trouble with this as theology is that when science fills these gaps in, Dodson said, it can squeeze the role of God out. Just as physics later found natural explanations for what Newton attributed to God's outreach, so biology may more fully explain complex cellular machinery in the future.

Dodson said his faith is unaffected by science because his God wouldn't necessarily leave traces - not in the fossil record or in DNA or anywhere in nature. For him and other scientific believers, absence of evidence is not evidence of God's absence.

In the past, intelligent design had less science to compete with, said Boston's Wildman, who identifies himself as an evangelical Protestant. In the fourth century B.C., Aristotle anticipated evolution and even proposed something like natural selection, but then rejected it because he thought nature was too complex to have emerged without an intelligent designer. In the 13th century, Wildman said, St. Thomas Aquinas argued persuasively that the natural world was intelligently designed by God.

The idea behind intelligent design pervades branches of all three major monotheistic religions - Judaism, Christianity and Islam, he said. All three have also sprouted branches in which faith does not rest on evidence for the divine in nature.

Theology and science

Some of today's proponents of intelligent design say it's not just theology - it's science. Todd Moody, a philosophy professor of St. Joseph's University, takes this view, arguing that the design theory debated in a Dover, Pa., courtroom is not necessarily a religious idea because it doesn't specify whether the designer is a supernatural power or not. "There's no way to know," he said. "It could be Klingons."

But the idea that aliens came to Earth and tweaked evolution doesn't qualify as science, said Michael Weisberg, a University of Pennsylvania philosophy professor. There's no evidence in its favor, nor is there any proposed way to get that evidence. "To move from speculation to scientific hypothesis, one has not just to assert something," he said, "but to show how it can be tested."

Though the current debate focuses on biology, plenty of other areas of science also gape with holes. Astronomy still can't explain the nature of the "dark matter" that seems to pervade the universe. Einstein's general relativity breaks down in describing the world at a subatomic scale.

Dodson says evolution is singled out not for anything it says about God but for what it seems to say about humanity. "I think the biggest problem for religious believers is the insistence that humans are an accident of an uncaring cosmos," he said.

Shuffling of genes through sex and mutation generates the variety from which natural selection can work. The chance encounter with an asteroid probably killed the dinosaurs, opening a new niche for the mammals that eventually spawned humanity. The late Stephen Jay Gould and other prominent scientists have said if we ran the clock back, evolution might produce a different mix of plants and animals and possibly a world with no human beings.

By adding a guiding hand, intelligent design allows its adherents to continue to believe humans were destined to be here - that we're here for a reason.

Boston's Wildman said his faith does not depend on the notion of humanity as central to the purpose of the universe. "It's a silly conceit and it makes human beings feel better to think that way - that the whole success of the universe turns on the success of the human project."

He said the tradition of ascribing so much importance to humanity goes back to the notion of a chosen people in the Old Testament. For Christians it comes from the belief that Jesus was God's son.

But such a God looks too small, too tribal and too humanlike, he said, to avoid getting pushed out of the ever narrowing gaps in science. That in turn can reinforce the notion that to be scientific you must be an atheist.

For him, a human-centered God is an approximation of the real thing, "and the real thing is beyond human comprehension."