At one level, the difference between a theist and atheist is simply a small gap of white space. But the divide between the two is also a chasm, a difference not just about sect and doctrine, or church vs. state, but about fundamental questions of the origin and purpose of life.

Not surprisingly, the two camps rarely try to understand each other — although in America they generally tend to live and work side by side without blowing each other up.

And so, on a recent afternoon, David Keller and Mark Hausam sat at a table in a downtown restaurant talking civilly about their upcoming debate entitled, "Does God Exist?" The debate, sponsored by Forum for Questioning Minds, takes place at 2 p.m. Sunday in the auditorium of the Salt Lake City Main Library, 210 E. 400 South. In a season of debates about candidates and school vouchers, in a city that is the headquarters of an international religion, "Does God Exist?" is perhaps the most audacious debate of the lot.

To Keller, an associate professor of philosophy at Utah Valley State College and director of the school's Center for the Study of Ethics, believing in God is an irrational act. For Hausam, adjunct philosophy instructor at Salt Lake Community College and an elder at Christ Presbyterian Church in Magna, nothing about the universe or human existence can be explained without God.

They met to talk about the debate's ground rules with moderator Deen Chatterjee, an associate professor of philosophy at the University of Utah. Chatterjee doesn't want some kind of namby-pamby discussion in which, for example, Keller just pokes some holes in Hausam's position. "To prove God doesn't exist," he cautions Keller, "you have to come up with separate arguments. You have to go beyond just saying, 'Mark can't prove God exists."'

The last time the Forum for Questioning Minds had a similar debate — about whether morality depends on God, also featuring Hausam and Keller — it drew a standing-room-only crowd. People were lined up at the audience microphone long after the debate was supposed to have ended.

The Forum draws well-informed, intelligent, mature adults, Chatterjee says. "Most have gone through their share of critical inquiry their whole lives. Some are humanists, and some are atheists."

"A theist gets thrown into the den," says Hausam with a wry smile.

Well, says Chatterjee, the debate probably also will draw lots of people from area churches. And many of the Questioning Mind folks would probably agree there is some form of "higher power or intelligence," he says.