Robin Collins offers three design arguments for the existence of God, with the primary fine-tuning argument contending that life or intelligent life depends for its existence on the fact that a number of physical parameters of the universe have (numerical) values that fall within a range of life-permitting values that is very narrow. Such fine-tuning entails, Collins argues, that the existence of life is much more surprising on naturalism than on theism.
Paul Draper rightly argues that fine-tuning gives us no reason to believe in a generic design hypothesis that tells us nothing about the motives of the designer. He also correctly notes that "The Case for Cosmic Design" does not establish the existence of God; but it nevertheless offers evidence for the existence of God. Fine-tuning is more surprising under the naturalistic single-universe hypothesis than under theism, and thus constitutes evidence for theism over the naturalistic single-universe hypothesis. When all of the evidence is considered, whether theism is objectively more likely to be true than the naturalistic single-universe hypothesis is an open question that depends upon a difficult assessment of the prior epistemic probability of theism. Since there are independent motivations for believing theism apart from fine-tuning and other design evidence, that evidence counts in favor of theism even if we cannot show that theism is true. Moreover, unless we have good reason to believe that the existence of evil is very improbable under theism, the combination of the fine-tuning data and the existence of evil supports theism over the naturalistic single-universe hypothesis. It is reasonable under the theistic hypothesis to think that the existence of limited, vulnerable moral agents is an overall good despite the evils that almost certainly would accompany the existence of such agents.
In his opening case Quentin Smith argues that the existence of the universe is self-explanatory because it is self-caused, and that this conclusion is inconsistent with theism. However, to be consistent with his principle that a causal explanation of each part of the universe logically explains the existence of the whole, and that the Big Bang caused the sequence of states following it, he must claim that the Big Bang provides an additional explanation of the sequence of states following it. But then the theist can claim that this is the sort of additional explanation that God provides for the existence of the universe, and that God is essential to providing a complete explanation of our universe, even though the universe contains no beginning point. Moreover, Smith's explanation of the existence of the universe may be fatally circular, or lead to an infinite regress where, no matter what part one starts with, the part of the universe doing the explaining is always further in need of an explanation—until one posits God to close the regress.