The phrase "I believe" -- which has such resonance at this time of year and sounds so certain and absolute -- is actually fraught with ambiguity. We see belief as a close cousin to faith. But Bill Galston, a University of Maryland political philosopher, says the relationship is much more complicated. "Belief, he says, "is an elastic word in a way that faith, curiously, isn't."

The word is so elastic that you can use it in the same conversation to imply that you really believe something -- or that you really don't. The religious believer who says, "I believe in God," is stating what for him or her is a profound truth. But the same person might ask moments later: "Does your child still believe in Santa Claus?" The question presumes what most of us grown-ups know: that those gifts really aren't put under the tree by a round, cheerful man in a red suit.

Peter Berger, the distinguished sociologist of religion, reminds us of the large distinction between belief and knowledge. "Some things I know, and some things I believe," he writes in A Far Glory: The Quest for Faith in an Age of Credulity. "Generally speaking, I don't have to believe what I know. Thus I know that 2+2=4. It makes little sense to say that I believe this. But if I have before me a closed box containing apples, I may say that I believe it contains four; I'm not sure, but I have some reason to think this is the number."

Yet there are also stronger uses of the word, Mr. Berger points out, as in, "I believe in democracy, or the integrity of my friend." Both statements express strong convictions for which a person may have strong but not indisputable evidence.

"When belief is used in ordinary parlance," says Mr. Galston, "it always raises the question of what are the grounds for belief." The idea is that the believer, whether in God or in the integrity of her friend, can make a series of arguments and provide evidence for the proposition.

But here things get complicated indeed. Someone who believes in God is commonly described as a person "having faith." But faith is not quite the same as belief. "As the language is publicly used, faith is a kind of conviction or commitment for which people may not be able to give grounds to those who don't share your commitment. The basis of faith may be less communicative than the basis of belief," says Mr. Galston

Sometimes, faith is used to refer to something more subjective than objective. The Rev. John Boyles, pastor of the Sixth Presbyterian Church in Washington, describes faith as "the feeling of belief." Thus is an intellectual category linked to an emotional, highly personal category.

The Rev. Paul Ojibway of the Graymoor Ecumenical and Interreligious Institute builds on this definition by distinguishing between merely "assenting to something" and "embracing it" firmly. "Belief can be notional until it is embraced, and then it becomes faith," he says.

But of course many people of faith insist that -- as Msgr. Francis Maniscalco of the National Conference of Catholic Bishops says -- "faith is reasonable and substantial and can be defended, that it's not just a feeling."

Making matters even more complicated is that, for many believers, faith is rooted not simply in a series of propositions, but in personal experience. This is true across religious traditions and, as Mr. Galston notes, is powerfully alive in the Old Testament. Jeremiah did not go out and prophesy until God told him to. Job had notoriously complicated, highly personal dealings with God. Similarly, contemporary Christians who say they have a "direct, personal relationship with Jesus Christ" are talking about something that, to them, is utterly real.

Yet unlike Jeremiah's specific prophecies or Christ's specific teachings, "a direct experience with God is not communicative," Mr. Galston says. That is why conversations between believers and unbelievers can be difficult. What one person knows from personal experience is not easily conveyed to someone who has not had a comparable experience.

Still, we feel a strong impulse to explain and rationalize our beliefs. We want others to understand and, perhaps, join us. Believers, therefore, are eager to find proof -- whether it be new evidence suggesting something like Noah's flood really happened or scientific backing for claimed miracles.

Achieving understanding and respect across the chasm that separates believers from nonbelievers may be one of the most difficult tasks of language.

Speaking for believers, Mr. Berger argues that his side should certainly understand those who don't. "God has not made it easy for human beings to believe, and the world provides good grounds for unbelief." But unbelievers can surely understand the logic, or appeal, of positing a meaningful universe with God at the center. In any case, in the spirit of the season, each side might embrace the belief that the other is acting -- and speaking -- in good faith.

What's the word? If you have suggestions or other ideas, write to Chatter, The Washington Post Magazine, 1150 15th St. NW, Washington, D.C. 20071, or e-mail Include your name, address and telephone number. E.J. Dionne will credit contributions he uses.