Robin Collins defends a more modest version of the fine-tuning argument that relies on a general principle of confirmation theory, rather than a principle that is contrived to distinguish events or entities that are explained by intelligent design from events or entities explained by other factors. Collins's version of the argument relies on what he calls the Prime Principle of Confirmation: If observation O is more probable under hypothesis H1 than under hypothesis H2, then O provides a reason for preferring H1 over H2. The idea is that the fact that an observation is more likely under the assumption that H1 is true than under the assumption H2 is true counts as evidence in favor of H1.
This version of the fine-tuning argument proceeds by comparing the relative likelihood of a fine-tuned universe under two hypotheses:
The Design Hypothesis: there exists a God who created the universe such as to sustain life;
The Atheistic Single-Universe Hypothesis: there exists one material universe, and it is a matter of chance that the universe has the fine-tuned properties needed to sustain life.
Assuming the Design Hypothesis is true, the probability that the universe has the fine-tuned properties approaches (if it does not equal) 1. Assuming the Atheistic Single-Universe Hypothesis is true, the probability that the universe has the fine-tuned properties is very small—though it is not clear exactly how small. Applying the Prime Principle of Confirmation, Collins concludes that the observation of fine-tuned properties provides reason for preferring the Design Hypothesis over the Atheistic Single-Universe Hypothesis.
At the outset, it is crucial to note that Collins does not intend the fine-tuned argument as a proof of God's existence. As he explains, the Prime Principle of Confirmation "is a general principle of reasoning which tells us when some observation counts as evidence in favor of one hypothesis over another" (Collins 1999, 51). Indeed, he explicitly acknowledges that "the argument does not say that the fine-tuning evidence proves that the universe was designed, or even that it is likely that the universe was designed" (Collins 1999, 53). It tells us only that the observation of fine-tuning provides one reason for accepting the Theistic Hypothesis over the Atheistic Single-Universe Hypothesis—and one that can be rebutted by other evidence.
The confirmatory version of the fine-tuning argument is not vulnerable to the objection that it relies on an inference strategy that presupposes that we have independent evidence for thinking the right kind of intelligent agency exists. As a general scientific principle, the Prime Principle of Confirmation can be applied in a wide variety of circumstances and is not limited to circumstances in which we have other reasons to believe the relevant conclusion is true. If the observation of a fine-tuned universe is more probable under the Theistic Hypothesis than under the Atheistic Single-Universe Hypothesis, then this fact is a reason for preferring the Design Hypothesis to Atheistic Single-Universe Hypothesis.
Nevertheless, the confirmatory version of the argument is vulnerable on other fronts. As a first step towards seeing one worry, consider two possible explanations for the observation that John Doe wins a 1-in-7,000,000 lottery (see Himma 2002). According to the Theistic Lottery Hypothesis, God wanted John Doe to win and deliberately brought it about that his numbers were drawn. According to the Chance Lottery Hypothesis, John Doe's numbers were drawn by chance. It is clear that John's winning the lottery is vastly more probable under the Theistic Lottery Hypothesis than under the Chance Lottery Hypothesis. By the Prime Principle of Confirmation, then, John's winning the lottery provides a reason to prefer the Theistic Lottery Hypothesis over the Chance Lottery Hypothesis.
As is readily evident, the above reasoning, by itself, provides very weak support for the Theistic Lottery Hypothesis. If all we know about the world is that John Doe won a lottery and the only possible explanations for this observation are the Theistic Lottery Hypothesis and the Chance Lottery Hypothesis, then this observation provides some reason to prefer the former. But it does not take much counterevidence to rebut the Theistic Lottery Hypothesis: a single observation of a lottery that relies on a random selection process will suffice. A single application of the Prime Principle of Confirmation, by itself, is simply not designed to provide the sort of reason that would warrant much confidence in preferring one hypothesis to another.
For this reason, the confirmatory version of the fine-tuning argument, by itself, provides a weak reason for preferring the Design Hypothesis over the Atheistic Single Universe Hypothesis. Although Collins is certainly correct in thinking the observation of fine-tuning provides a reason for accepting the Design Hypothesis and hence rational ground for belief that God exists, that reason is simply not strong enough to do much in the way of changing the minds of either agnostics or atheists.