The struggle: a 17th-century painting by The French artist Nicolas Lebrun depicts "The Triumph of Faith."

One might have expected the forces of Reason to be a bit weary after a generation of battling postmodernism and having its power and authority under constant scrutiny. Reason's battles, though, continue unabated. Only now it finds its opposition in the more unyielding claims of religious faith. This latest conflict is over seemingly incompatible ways of knowing the world. It is a conflict between competing certainties: between followers of Faith, who know because they believe, and followers of Reason, who believe because they know.

This battle echoes others taking place between fundamentalist terror, which claims the authority of Faith, and Western modernity, which claims the authority of Reason. But some of Reason's combatants — as if reading from the postmodernist strategy book — are also challenging the heritage of the West, arguing that it, too, has been riddled with absolutist faith, that the reasoned achievements of the Enlightenment are still under threat and that a new understanding of the past must take shape, in which Reason's martyrdom and trials take center stage.

One motivation for Reason's latest salvos is political. A Gallup poll last year said that about 40 percent of Americans considered themselves evangelicals or born-again Christians. They include the president, the attorney general, the speaker of the House and the House majority leader.

Critics of the Bush administration's policies sometimes cite such beliefs as evidence of the administration's potential fundamentalism and intolerance. In the recent book "A Devil's Chaplain" (Houghton Mifflin, $24), for example, Richard Dawkins, the Oxford University evolutionary biologist, worries about American responses to the attacks of 9/11 because "the United States is the most religiose country in Christendom, and its born-again leader is eyeball to eyeball with the most religiose people on Earth."

Mr. Dawkins has long been a harsh critic of religion, which he considers a form of infectious virus that readily replicates, spreading its distortions. Last summer he lobbied in The Guardian for adopting "bright" as a noun to mean atheist (as in "I'm a bright. You're a bright").

The philosopher Daniel C. Dennett echoed his urgings in an Op-Ed article in The New York Times. Mr. Dawkins and Mr. Dennett argue that brights are a beleaguered group confronting a growing religious right; they urge brights to emerge from their closet and boldly proclaim their identity.

"So, what's the opposite of a bright?" Mr. Dawkins imagines someone asking,

"What would you call a religious person?" "What would you suggest?" he coyly responds.

There are of course approaches that are less blunt and more liberal minded, but the sense of embattlement and polemic has become familiar. In the recent book "The Closing of the Western Mind" (Knopf, $30), for example, Charles Freeman argues that Western history has to be retold. Over the course of centuries, he points out, the ancient Greeks recognized the importance of reason, giving birth to the techniques of modern science and mathematics, and establishing the foundations of the modern state. But then, he writes, came "the closing of the Western mind."

In the fourth and fifth century, he writes, the Greek intellectual tradition "was destroyed by the political and religious forces which made up the highly authoritarian government of the late Roman empire," particularly with the imposition of Christian orthodoxy. For a millennium doctrine ruled. Reason became heresy.

It is precisely this sort of heresy that Jennifer Michael Hecht celebrates in "Doubt: A History" (HarperSanFrancisco, $27.95), which outlines the views of those who rejected dominant doctrines of faith or proclaimed disbelief in the existence of God. Her loosely defined roster of doubters ranges from the ancient Greeks to Zen Buddhists, along with such familiar figures as Galileo, Hobbes, Gibbon, Tom Paine and Thomas Jefferson.

Ms. Hecht is more generous than Mr. Dawkins, noting that just as there are believers who "refuse to consider the reasonableness of doubt," so, too, there are nonbelievers who "refuse to consider the feeling of faith." But her sympathies are committed to the doubters, including such unusual figures as the Islamic philosopher and physician Abu Bakr al-Razi (854-925) and Annie Besant, who wrote a "Gospel of Atheism" in 1876, helped reform London schools with free meals and medical care, and later in life became a theosophist and a translator of the Bhagavad-Gita.

Ms. Hecht's goal is to provide an affirmative history for doubters. "To be a doubter," she writes, "is a great old allegiance, deserving quiet respect and open pride."

What, though, is the nature of this doubt? Its demarcation from faith is not as precise as these descriptions suggest. Doubt can become a rigid orthodoxy in its own right. In contemporary life, as Ms. Hecht seems to know, doubt has become almost axiomatic (as if it were a matter of faith).

Meanwhile faith itself is riddled with doubt. As Ms. Hecht points out, many religious texts (like Job or Augustine's "Confessions") are also accounts of doubt.

Yet in these arguments faith is often portrayed as monolithic, a host for intolerance and inquisition. And while that has been part of many religions' history — and is, as Mr. Freeman shows, part of the history of Christianity — the nature of faith is far more complex.

In his recent book, "The Transformation of American Religion" (Free Press, $26) for example, the sociologist Alan Wolfe suggests that evangelical Christians in the United States cannot be thought of as they once were. Religion, he argues, has been transformed by American culture to become therapeutic, individualistic and less interested in doctrine than in faith.

Nor is faith always unreasonable. Religious beliefs were fundamental to the abolition of slavery in the 19th century and to the civil rights movement in the 20th. Faith may even be latent in some of science's triumphs, inspiring such figures as Newton and Kepler. The conviction that there is an order to things, that the mind can comprehend that order and that this order is not infinitely malleable, those scientific beliefs may include elements of faith.

Reason also has its own problems. Isaiah Berlin argued that the Enlightenment led to the belief that human beings could be reshaped according to reason's dictates. And out of that science of human society, he argued, came such totalitarian dystopias as the Soviet Union.

Reason then, has its limits. The philosopher Robert Fogelin's new book, "Walking the Tightrope of Reason" (Oxford, $22) is subtitled "The Precarious Life of a Rational Animal" because, he argues, reason's own processes negotiate a precipice. Mr. Fogelin quotes Kant, who described a dove who "cleaving the air in her free flight, and feeling its resistance, might imagine that its flight would be still easier in empty space."

Failing to understand what keeps her aloft and taking a leap of faith, the dove might set off in "empty space" — a vacuum — and plummet. But reason might lead to the same end: if something offers resistance then logically can't one proceed more easily if it is eliminated? So why not try?

The problem is that the bird can never fully comprehend the medium through which it experiences the world. In many ways, Kant argued, neither could the mind. Reason is still the only tool available for certain knowledge, but it also presents questions it is unable to answer fully.

Some of those questions may remain even after contemporary battles cease: how much faith is involved in the workings of reason and how much reason lies in the assertions of faith?

Copyright 2003 The New York Times Company. Reprinted from The New York Times, Arts & Ideas, of Saturday, December 20, 2003