One of the blurbs on the back cover of “Breaking the Spell” proclaims that Daniel Dennett approaches the subject of religion in the same way a Martian would. Whether you find this grounds for praise is a good test of whether you will enjoy Mr. Dennett’s earnest, impassioned, but finally confused book. For Mr. Dennett himself, approaching religion Martianwise – that is, ostensibly without personal investment or preconception – is the only way to bring to bear on this most explosive of subjects the logic and rigor of science. In particular, Mr. Dennett, known for his popularizing works on evolutionary biology (including “Darwin’s Dangerous Idea”), argues that evolution offers a good way to explain why human beings are so inveterately religious. By showing that we evolved to believe, Mr. Dennett hopes to reduce belief to the status of an ordinary human disposition, no more mysterious than our appetite for sweets or our sexual drives. And from there, he hopes, it will be only a short hop to demolishing belief altogether, as a vestige of our prehistory that has become maladaptive in an advanced civilization.

This movement from analysis to prescription – from explaining faith as a Darwinian relic to abolishing it as a dangerous holdover – is not incidental to “Breaking the Spell.” As the title itself suggests, it is the very reason Mr. Dennett wrote the book in the first place. He candidly describes him self as a “godless philosopher” and has invented an obviously value laden term, “bright,” to describe people like himself who are proudly emancipated from religion. He is careful not to pronounce outright on the existence of God or the truth of any given religion – preferring to argue that what religion needs is not affirmation or denial, but study – but there is no doubt that Mr. Dennett believes the world would be better off if religion disappeared tomorrow. If his actual assertions leave any uncertainty about this, his metaphors and images do not: On the very first page, for instance, he compares human religions to Dicrocelium dendriticum, a parasite that lives in the brain of ants and compels them to irrational, self-destructive behavior.

The problem with “Breaking the Spell” (Viking, 464 pages, $25.95) is not this frank hostility to religion. On the contrary, there is a long, honorable, and thrilling tradition of atheistic polemics, from Voltaire to Nietzsche and beyond. If anything, one wishes Mr. Dennett were more familiar with this literature and had learned its most important lessons. If he had, perhaps his own attacks on religion and religious people would not sound so much like the complacent broadsides of the village atheist. For the best atheists agree with the best defenders of faith on one crucial point: that the choice to believe or disbelieve is existentially the most important choice of all. It shapes one’s whole understanding of human life and purpose, because it is a choice that each of us must make for him or herself. To impress on a man the urgency of that choice, Kierkegaard wrote, it would be useful to “get him seated on a horse and the horse made to take fright and gallop wildly … this is what existence is like if one is to become consciously aware of it.”

Mr. Dennett would have benefited from a ride on Kierkegaard’s horse. For what dooms his book, not just in literary but in logical terms, is his complete failure to recognize the existential demand of religion. “I decided some time ago,” he writes, “that diminishing returns had set in on the arguments about God’s existence,” and so he leaves God out of his argument entirely. Instead, Mr. Dennett writes about religion as a purely social and empirical phenomenon: “I propose to define religions as social systems whose participants avow belief in a supernatural agent or agents whose approval is to be sought.” Starting with this definition, Mr. Dennett proceeds to analyze religion anthropologically, as a behavior, an institution, and an aesthetic taste. But because the definition so completely misses the actual substance of religious experience, none of Mr. Dennett’s subsequent arguments, from the plausible to the frankly speculative, has the wished-for effect of making religion questionable.

This is not to suggest that Mr. Dennett’s arguments about the origins of religion are iron-clad even on their own terms. On the contrary, in his application of evolutionary biology to religious practice, Mr. Dennett falls prey to the common fallacy of neo-Darwinists, which might be called the genealogical fallacy: the assumption that a human phenomenon can be fully explained by an explanation of its origins. Drawing on recent, speculative work by evolutionary theorists, Mr. Dennett sketches a picture of how religion might have arisen as a naturally selected adaptation to the early human environment. Perhaps, he suggests, credulous Homo sapiens had a higher survival rate because they were more susceptible to the placebo effect, and thus more likely to be “cured” of diseases by the ministrations of a shaman.

But even if such Darwinian just-so stories were confirmed – and it is not immediately clear how they could be tested – it would make no difference to the fact of religious experience. Mr. Dennett believes that explaining religion in evolutionary terms will make it less real; that is the whole purpose of his book. But this is like saying that because water is made of two parts hydrogen and one part oxygen, it is not really wet; or because the color red represents a certain frequency of light, it is not really red. To human beings, the wetness of water, the redness of red, is existentially prior to their physical composition. Just so, the reality of religious experience cannot be abolished by explaining it as an adaptation to our prehistorical environment.

Mr. Dennett is left, then, with two aporias. The first is that, as we have all learned, you cannot move from an “is” to an “ought”: in this case, from the assertion that religion evolved to the prescription that we stop practicing it. Any ethical exhortation, and that is what “Breaking the Spell” boils down to, must employ ethical arguments, which means arguments about truth and human flourishing. These are the kinds of arguments that serious atheists have dared to make: that religious belief is a dereliction of our ethical responsibility, an affront to our intellectual honor. Mr. Dennett surely believes this as well, but his failure to argue it leaves his atheism looking like just another prejudice.

The second, and more important, dilemma for Mr. Dennett is that there are kinds of truth the positivist cannot measure. At the heart of organized religion, whether one accepts or rejects it, is the truth that metaphysical experience is part of human life. Any adequate account of religion must start from this phenomenological fact. Because Mr. Dennett ignores it, treating religion instead as at best a pastime for dimwits, at worst a holding cell for fanatics, he never really encounters the thing he believes he is writing about. The obvious thing to say about “Breaking the Spell” is that it will irritate believers and delight nonbelievers, but that is not, or should not be, the case. In fact, it should irritate both, for it trivializes matters that have engaged the best human minds for thousands of years.