Among a heap of books claiming that science proves God’s existence emerges one that computes a probability of 67 percent

In his 1916 poem “A Coat,” William Butler Yeats rhymed: “I made my song a coat/Covered with embroideries/Out of old mythologies/From heel to throat.”

Read “religion” for “song,” and “science” for “coat,” and we have a close approximation of the deepest flaw in the science and religion movement, as revealed in Yeats’s denouement: “But the fools caught it,/Wore it in the world’s eyes/As though they’d wrought it./ Song, let them take it / For there’s more enterprise/In walking naked.”

Naked faith is what religious enterprise was always about, until science became the preeminent system of natural verisimilitude, tempting the faithful to employ its wares in the practice of preternatural belief. Although most efforts in this genre offer little more than scientistic cant and religious blather, a few require a response from the magisterium of science, if for no other reason than to protect that of religion; if faith is tethered to science, what happens when the science changes? One of the most innovative works in this genre is The Probability of God (Crown Forum, 2003), by Stephen D. Unwin, a risk management consultant in Ohio, whose early physics work on quantum gravity showed him that the universe is probabilistic and whose later research in risk analysis led him to this ultimate computation.

Unwin rejects most scientific attempts to prove the divine — such as the anthropic principle and intelligent design — concluding that this “is not the sort of evidence that points in either direction, for or against.” Instead he employs Bayesian probabilities, a statistical method devised by 18th-century Presbyterian minister and mathematician Reverend Thomas Bayes. Unwin begins with a 50 percent probability that God exists (because 50–50 represents “maximum ignorance”), then applies a modified Bayesian theorem:

The probability of God’s existence after the evidence is considered is a function of the probability before times D (“Divine Indicator Scale”): 10 indicates the evidence is 10 times as likely to be produced if God exists, 2 is two times as likely if God exists, 1 is neutral, 0.5 is moderately more likely if God does not exist, and 0.1 is much more likely if God does not exist. Unwin offers the following figures for six lines of evidence: recognition of goodness (D = 10), existence of moral evil (D = 0.5), existence of natural evil (D = 0.1), intranatural miracles (prayers) (D = 2), extranatural miracles (resurrection) (D = 1), and religious experiences (D = 2).

Plugging these figures into the above formula (in sequence, where the P_after figure for the first computation is used for the P_before figure in the second computation, and so on for all six Ds), Unwin concludes: “The probability that God exists is 67%.” Remarkably, he then confesses: “This number has a subjective element since it reflects my assessment of the evidence. It isn’t as if we have calculated the value of pi for the first time.”

Indeed, based on my own theory of the evolutionary origins of morality and the sociocultural foundation of religious beliefs and faith, I would begin (as Unwin does) with a 50 percent probability of God’s existence and plug in these figures: recognition of goodness (D = 0.5), existence of moral evil (D = 0.1), existence of natural evil (D = 0.1), intranatural miracles (D = 1), extranatural miracles (D = 0.5), and religious experiences (D = 0.1). I estimate the probability that God exists is 0.02, or 2 percent.

Regardless, the subjective component in the formula relegates its use to an entertaining exercise in thinking — on par with mathematical puzzles — but little more. In my opinion, the question of God’s existence is a scientifically insoluble one. Thus, all such scientistic theologies are compelling only to those who already believe. Religious faith depends on a host of social, psychological and emotional factors that have little or nothing to do with probabilities, evidence and logic. This is faith’s inescapable weakness. It is also, undeniably, its greatest power.