From The Washington Post's Book World/washingtonpost.com

Reviewed by Wray Herbert

In 1877, the Italian astronomer Giovanni Schiaparelli was looking at Mars through his new telescope, and he noticed intricate etchings in the equatorial region of the planet's surface. Schiaparelli called these lines canali, by which he probably meant something like "gullies" or "grooves," but his coinage got wrongly translated into English as "canals." It was a regrettable linguistic slip.

The idea of Martian canals grabbed the imagination of American astronomer Percival Lowell, scion of the famous Boston Lowell clan, who spun out an elaborate story of a Martian civilization with a central planetary government and the technological wizardry to engineer a massive system of aqueducts. Lowell even used his own Arizona observatory to identify the Martian capital, called Solis Lacus.

There are no canals on Mars. No cities either, and no government. Indeed, no signs of past life whatsoever, as we know today. All of this was an elaborate phantasm of Lowell's fertile mind, yet as late as the 1950s, popular culture was saturated with imagery of Martians as a technologically advanced extraterrestrial race.

The late Carl Sagan used the misbegotten tale of Martian engineers, in his 1985 Gifford Lectures in Natural Theology at the University of Glasgow, as a cautionary tale about the power of belief and yearning to trump science and reason. The Cornell University astrophysicist, Pulitzer Prize-winning author and TV personality was alarmed by the persistence of such magical thinking even into the late 20th century, despite tremendous scientific progress in understanding both human nature and the cosmos. He used the prestigious lecture series (collected here for the first time by his widow and long-time collaborator, Ann Druyan) as an opportunity to challenge the evidence for everything from the Bermuda Triangle to UFOs to angels and deities. But just as important, he used the lectures to spell out his views on the common ground shared by science and spirituality.

Sagan does not deny the existence of God. Nor does he affirm it. As he quips in the lively Q&A section appended to the lectures, "Absence of evidence is not evidence of absence." What Sagan does do is insist on the primacy of scientific method and scientific evidence, and he holds the many and various "proofs" of God's existence up to these scientific standards. Most are found wanting. But Sagan is not harsh in his critiques of religious thought; he is more perplexed by theology's narrow and unimaginative vision.

Why would an all-powerful God work only on a local (and recent) project like the Earth when there is a vast, 15-billion-year-old universe out there, with countless galaxies containing countless stars and the possibility of countless worlds? Why didn't God let us know about quantum mechanics and natural selection and cosmology from the get-go? And why would theologians insist on such a provincial version of the creation and God's imagination?

Sagan is not being flip or heretical, though he is intellectually playful and obviously likes the fray. Sagan took his own spirituality seriously -- indeed, he defined science as "informed worship." The closest he comes to articulating his own view of God is to describe admiringly the philosophies of Spinoza and Einstein, who basically considered God the sum total of all the laws of physics. These laws, he emphasizes again and again, govern not just the Earth and humanity but every solar system and every star and every galaxy. They are not local ordinances.

Central to Sagan's personal search for the existence of God is the question of other life in the universe. For him, the requirements for proof of extraterrestrial intelligence are essentially the same requirements for proof of angels or demigods or a God. Sagan spends much of one lecture on the so-called Drake equation, which is a way of estimating the number of technologically advanced civilizations in the galaxy. The equation incorporates several values, including the rate of star formation, the fraction of stars with planets around them, the fraction likely to have evolved intelligent life and so forth.

The confounding value is L, which stands for the average lifetime of a technologically advanced civilization. If you plug in an optimistic value, assuming such civilizations are long-lived, then the equation predicts that there are millions of intelligent societies out there, and likely one just a few hundred light years away. That's a long way by spaceship, but really right next door if you're going at the speed of light, which means that our radio telescopes should be able to pick up signals from other advanced beings who want to contact us.

But what if the value of L is low? What if technologically advanced civilizations don't last very long on average? If highly intelligent races tend to perish quickly, then the Drake equation predicts not millions of such civilizations but one. Us. We are alone.

In 1985, Sagan was especially concerned about the 55,000 nuclear warheads strategically placed around the globe, threatening to make Earth a cosmological loser. A recurring theme in these lectures is that our scientific prowess is double-edged, revealing the awesomeness of nature while landing us in great peril.

Yet this is not a dour book. Far from it. Sagan was fundamentally an optimist, and The Varieties of Scientific Experience is mostly a joyful, celebratory meditation on nature and the expansiveness of the human spirit. The volume was published on the 10th anniversary of Sagan's death in December 1996. For those who have sorely missed his clear and wise voice, it will be received as a gift.