[In the following essay, Adler discusses Sagan's unshakable faith in science over religion even in the face of fatal illness.]

A man of science, Carl Sagan didn't want prayers; he wanted proof. He died still waiting for evidence.

Carl Sagan, the famous scientist and author, never asked for anyone to pray for him, although in his final illness many people did anyway. For two years prayers for his health filled the great Cathedral of St. John the Divine in New York. They rose (if prayers do rise) to the heaven Sagan had never seen in all his years of searching the sky, and were heard (if prayers are heard) by the God Sagan never called on. And God (if he exists) let Sagan die anyway, late last year, at the untimely age of 62, leaving behind a wife, five children and much unfinished work on the earth he loved so well. But he died in what amounted, for him, to a state of grace: resisting the one temptation to which almost everyone submits in the end, the temptation to believe.

Not that the Kingdom of Heaven held no interest for Sagan, an astronomer who found the solar system too confining for his speculations on cosmic origins, human consciousness and evolution. For most of the last decade of his life he engaged in a wide-ranging dialogue with religious leaders on the question whose answer held the potential to put either preachers or cosmologists out of business: does God exist? He argued the negative, although his formal position was agnostic, awaiting proof. On the other side were primarily mainstream, liberal Protestant clerics, such as the Rev. James Parks Morton, then dean of St. John the Divine, and the Rev. Joan Brown Campbell, general secretary of the National Council of Churches, whom Sagan met in the environmental movement. But he also exchanged views with believers of a more conservative bent, such as Robert Seiple, head of the Christian relief organization World Vision. Sagan was fascinated by the phenomenon that educated adults, with the wonders of science manifest all around them, could cling to beliefs based on the unverifiable testimony of observers dead for 2,000 years. "You're so smart, why do you believe in God?" he once exclaimed to Campbell. She found this a surprising question from someone who had no trouble accepting the existence of black holes, which no one has ever observed. "You're so smart, why don't you believe in God?" she answered.

Sagan never set out to finish the work of the Enlightenment singlehandedly. "I started out very much enjoying the idea of an omnipotent, omniscient, benevolent God who like a benign parent was watching out for me," he wrote to one of his correspondents. "I was brought to skepticism by the slow realization that the 'evidence' [for religion] is anecdotal … But if there is evidence of such a God, or any other God, I feel it is my responsibility to try and know about it."

For that matter, Sagan's early career suggested he had the potential to become as big a public nuisance as Shirley MacLaine. Twenty years ago, as a scientist on the Viking Mars probe project, he was known as a particularly visionary believer in the possibility of life on other planets, to the point where some of his colleagues at NASA considered him a flake. But it was only the possibility he believed in. When Mars landers found nothing but rocks, he accepted the evidence—as, years earlier, he had himself published a seminal paper showing that Venus was too hot to support life. Thus "Carl's major contributions to science each flew in the face of his own most cherished expectations," remarks his wife and sometime collaborator, Ann Druyan. He thought believers should be just as willing to jettison their beliefs in response to evidence. A religion whose highest sacrament is heresy might have won Sagan's allegiance, but he never found one.

Sagan developed many of these ideas in his 1995 book, The Demon-Haunted World, a defense of science against the superstitious nonsense that he saw in American culture, from alien abductions to "recovered memories" of satanic ritual abuse. He managed to suggest, with considerable circumspection, that the evidence for most religion is not very much stronger. Sagan's insistence on dealing only with "evidence" put his correspondents at a seeming disadvantage, at least until the time Campbell asked him, "Carl, do you believe in love?"

"And he said, 'Of course I do.' He was very much in love with his wife. And I said, 'Can you prove love exists?' And at first he said, 'Well, certainly,' but eventually he agreed that love, like faith, has something unprovable at its core, but that doesn't mean it doesn't exist."

But that was still a long way from accepting the claims of organized religion. Sagan reserved particular scorn for petitionary prayer, which by its very utterance renders God's qualities of omnipotence, omniscience and benevolence mutually contradictory. Does God need to be reminded that someone is sick, Sagan asked. Or does he know, but he won't do anything about it unless someone else asks him to?

Of course, many believers have wrestled with these same questions. Morton began praying for Sagan after he was diagnosed with myelodysplasia, a disease related to leukemia, in the winter of 1995. "That prayer works is very clear," Morton says, "although how, I don't have a clue." If God had actually cured Sagan with a visible miracle, as in the Bible, Morton would have been only a little less astonished than the patient himself. "Look at it psychologically," he says. "It helped keep his spirits going. If your spirits are up, your body works better. Any kind of mechanistic notion of changing the molecules, that's nonsense."

But Seiple, from a very different religious tradition, had a more direct appreciation of the power of prayer. In a letter shortly before Sagan was to undergo a marrow transplant, he asked about his condition so he could "pray more intelligently" for a cure. "I have already begun to pray," he reassured Sagan, "and, as I have tried to persuade you, prayer has been the most necessary part of bridging the gap between the divine and our humanity."

Sagan was eventually to have three bone-marrow transplants, and by last summer seemed to be recovering. Campbell had dinner with him in the fall and said, "I think you're going to make it," to which he replied, smiling, "I'm praying I'm going to make it." Then he contracted pneumonia, a side effect of his radiation treatment. His friends prayed harder, but Sagan never wavered in his agnosticism.

"There was no deathbed conversion," Druyan says. "No appeals to God, no hope for an afterlife, no pretending that he and I, who had been inseparable for 20 years, were not saying goodbye forever."

Didn't he want to believe? she was asked.

"Carl never wanted to believe," she replies fiercely. "He wanted to know."