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.
Robin Collins argues that three facts implicate a designer of the universe--that life depends upon the precise tuning of physical constants, that the laws of physics show evidence of beauty, and that the universe is intelligible. But Collins' case is pervaded by vague arguments which shift between defending theism specifically and defending a more generic design hypothesis. This provides the appearance of having all of the advantages of the generic design hypothesis, such as greater initial plausibility, while masking the implication that intelligent life is just as unlikely given design with unspecified motives as it is given "chance." If design is to provide us with any expectations at all about what the world would be like, Collins has to defend theism in particular throughout. Moreover, while on single-universe naturalism the existence of anything as impressive as human beings may be very unlikely, on theism the existence of intelligent beings as unimpressive and flawed as humans is also very unlikely. And given that human beings do exist, single-universe naturalism, but not theism, entails that they exist in this particular universe.
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.
In his opening argument, Quentin Smith argued that universe explains its own existence, without remainder, even if the universe has a finite age, for the state of the universe at any particular moment is sufficiently caused by all of its preceding states. Since this complete explanation makes no reference to God, Smith argued, insofar as God is by definition a part of any complete explanation of the universe, God does not exist. In his response, Robin Collins cited the flight of a cannonball as a counterexample to Smith's line of reasoning, but the counterexample is not analogous; unlike the universe, the flight of the cannonball does not have a historically complete explanation in terms of earlier parts of that flight. Being charitable to Collins, however, it is possible that although the universe has no first moment in physical time, it may in some metaphysical time series, allowing one to make room for God in a complete metaphysical explanation of the universe. Smith's argument, then, might not demonstrate the nonexistence of God, but it nevertheless provides a probabilistic argument against the existence of God. And on Collins' own "likelihood principle," the fact that our best scientific theory of the origin and evolution of the universe supports a self-caused universe is much more likely on naturalism than on theism, and thus provides very strong evidence for naturalism over theism.