Default logic
Stephen D. Unwin is a physicist and author best known for his book, The Probability of God. Unwin is a graduate of Imperial College London and received his doctorate in theoretical physics from the University of Manchester for his research in the field of quantum gravity. Formerly the technical attaché to the United States Department of Energy for the British government, he is president of his own consulting firm, specializing in risk management for various Fortune 100 clients.
In his book, Unwin argues that a mathematical equation developed by Thomas Bayes can be used to calculate the probability that God exists. He does not make the claim that application of this method produces an absolute probability on which everyone would agree, but that it provides a systematic way of ordering one's ideas, weights of belief, and uncertainties in order to determine their implications regarding the probability that God exists.
Unwin employs Bayesian probabilities, a statistical method devised by Reverend Thomas Bayes. He begins with a 50 percent probability that God exists (arguing that 50–50 represents "maximum ignorance"), then applies a modified Bayesian theorem:
In this model, 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 Pafter-figure for the first computation is used for the Pbefore-figure in the second computation, and so on for all six Ds), Unwin concludes: "The probability that God exists is 67%." But then he notes that "this number has a subjective element since it reflects my assessment of the evidence." Unwin's comment refers to his estimates of the various "D" values used to obtain his estimate, whose values would be disputed by many.