[Below, Ayala presents a favorable review of The Demon-Haunted World, but disagrees with Sagan's reductionist view of scientific truth.]

In 1961 while driving at night in the White Mountains, Betty and Barney Hill sighted a bright object in the sky that seemed to follow them. Fearing for their safety, they left the main highway and took narrow roads, arriving home two hours later than they had expected. The experience prompted Betty to read a book that described UFOs as spaceships navigated by little men from other worlds, who sometimes abducted humans. Soon thereafter, she began experiencing a repetitive nightmare in which she and Barney were abducted and taken aboard a UFO. In a few days they were describing a pancake-like UFO with uniformed figures visible through the craft's windows. This and other motifs of the Hills' account are similar to those found in the 1953 motion picture Invaders from Mars. Later, Barney described the enormous eyes of the aliens, just 12 days after aliens were so portrayed in an episode of the television series The Outer Limits. The Hills' story was made into a 1975 movie purporting that short, grey alien abductors are among us in the psyches of millions of people.

Carl Sagan tells that he met with the Hills for several hours, and writes: "There was no mistaking the earnestness and sincerity of Betsy and Barney, and their mixed feelings about becoming public figures." Nevertheless, there are many reasons to doubt that the events described by the Hills happened in the world outside their mental experience. Sagan pursues this and other case histories of a demon-haunted world with the sure-footedness of a well-informed observer, the narrative skills of an engaging raconteur, and the subtle destructiveness of an experienced educator.

We encounter the Man in the Moon, the Face of Mars, the Dragon in the Garage, and countless stories of UFO sightings, abductions by aliens, and miraculous apparitions. Sagan meticulously debunks each story by noting absence of verifiable information, uncovering suspicious coincidence of conditioning circumstances, and pursuing other lines of reasoning that would persuade an impartial reader that the claimed experiences resulted from dreams and hallucinations, rather than from events in the outside world. We are provided a long list of typical offerings at the table of pseudoscience and superstition: astrology, the Bermuda Triangle, Big Foot, the Loch Ness monster, extrasensory perception, bleeding statues, divining rods, pyramidology, palmistry, numerology, faith-healers, Ouija boards, and much more.

Sagan tackles antiscience, in addition to pseudoscience. Science has been under attack for centuries, he proclaims. The nemeses of our time are postmodernists and deconstructionists (he does not use the latter term) who deny the objectivity of science. "Some even allege," he laments, "it's entirely subjective, as is, they say, history." Sagan sees that historical accounts are often self-promotional; what really happened is colored by subjective biases. Scientists also have biases and breathe the prevailing prejudices of their environment. But science is a collective enterprise endowed with the error-correcting process of empirical testing. In science, "you can rerun the event as many times as you like, examine it in new ways, test a wide range of alternative hypotheses." Scientists are biased and commit mistakes, but "Science thrives on errors, cutting them away one by one. False conclusions are drawn all the time, but they are drawn tentatively. Hypotheses are framed so they are capable of being disproved."

Science is the candle in the dark of the book's title, and Sagan seeks to characterize its distinctive attributes. One, according to him, is that science can predict the future. "Not every branch of science can foretell the future—paleontology can't—but many can and with stunning accuracy. If you want to know when the next eclipse of the Sun will be, you might try magicians or mystics, but you'll do much better with scientists." Here Sagan and I part company. In the matter of foretelling the future I don't think that astronomy is the rule and paleontology the exception, but the other way around.

Scientists predict the course of rockets and the statistical distribution of populational events, but that is in my book a long way from foretelling the future. Yet I think it correct to assert that "science is predictive," as the slogan goes. Indeed, being predictive of unknown facts is essential to the process of empirical testing of hypotheses, the most distinctive feature of the scientific enterprise. A hypothesis is tested empirically by ascertaining whether or not predictions about the world of experience derived as logical consequences from the hypothesis agree with what actually becomes observed. What is being predicted in this process is an unknown state of affairs, not necessarily a future event. And the prediction is made by logical deduction from the hypothesis.

The hypothesis that chimps are more closely related to humans than to gorillas is tested by examining DNA segments from each species, which the hypothesis predicts will be more similar between human and chimp than between chimp and gorilla. The evolutionary divergence of humans, chimps, and gorillas happened in the distant past, and their DNA is already there. When I now examine it, I test my hypothesis. Sagan has gone astray by failing to distinguish prediction in the logical sense (by deduction) from prediction in the temporal sense (foretelling the future), which is not an essential feature of science.

Sagan has much to say about reductionism as a distinctive feature of science that accounts for much of its success. Science seeks understanding of an event or process by investigating its component elements and underlying processes. The success of this analytical mode is unquestionable (although the antithetical mode is also successful; in matters of research strategy, what counts is success, not how we get there). We might call this kind of research strategy "methodological" or "strategical" reductionism.

But Sagan claims much more. He writes: "Until the middle twentieth century, there had been a strong belief … that life was not 'reducible' to the laws of physics and chemistry, that there was a 'vital force,' an 'entelechy,' a tao, a mane that made living things go." He tells the story of the 18th-century chemist Joseph Priestley, who found no difference in the weight of a mouse just before and after its death. Nothing had departed with death, at least nothing that could be weighed. Most scientists would agree, I suppose, with this kind of reductionism (let's call it "ontological" or "physical"). Living things are exhaustively composed of atoms, if we remove all the atoms that make up a mouse body, nothing is left. But accepting this kind of reductionism does not in any way entail the claim that biology is reducible to the laws of physics and chemistry. This is an epistemological claim, which can be shown to be mistaken by simply pointing out that the origin of species or symbolic language (or the majority of the subjects worth of investigating in biology and other disciplines) cannot be explained by the laws of physics or chemistry.

Sagan might state a conviction that such reductionism (of, say, the laws of biology to the laws of physics) will be accomplished in the future. But this is a statement of faith. The late philosopher Karl Popper argued that complete epistemological reduction of a discipline to another is impossible in principle. Sagan asks rhetorically: "Why should some religious people oppose the reductionist program in science, except out of some misplaced love of mysticism?" Popper's opposition to the (epistemological) reductionist program in science was certainly not religiously motivated, nor was he particularly appreciative of mysticism.