 Some argue, however, that the truth of theism is ‘evidentially ambiguous’—that is, that our total available evidence is equally viably interpreted from both a theist and a naturalist/atheist perspective (Hick 1966 and 1989; Davis 1978; Penelhum 1995; McKim 2001). This thesis of evidential ambiguity may be supported as the best explanation of the diversity of belief on religious matters, and/or of the persistence of the debate about theism, with philosophers of equal acumen and integrity engaged on either side. Or the ambiguity may be considered systematic—for example, on the grounds that both natural theological and natural atheological arguments fail because they are deeply circular, resting on implicit assumptions acceptable only to those already thinking within the relevant perspective. (In relation to Swinburne’s Bayesian natural theology, in particular, this objection surfaces in criticism of assumptions about how to set the prior probabilities implicated in calculations of, for example, theism’s probability on the evidence of the ‘fine-tuning’ of the Universe’s basic physical constants, or of the probability, on all our evidence, of the truth of the Resurrection.) If the ambiguity thesis is correct, then—assuming evidentialism—firmly held theistic belief will fail to be reasonable.