Prior probabilities and confirmation theory: A problem with the fine-tuning argument

Abstract

Fine-tuning arguments attempt to infer the existence of God from the presumably improbable fact that the universe is able to support life. Life would not be possible if any of approximately two-dozen fundamental laws and properties of the universe had been even slightly different. While there are different versions of the argument, they all attempt to show that the surprising fact that the universe has every one of these necessary properties supports the proposition that there exists a perfect God who created the universe for the purpose of supporting life. In this essay, I consider two versions of the fine-tuning argument that rely on different constructions of the following general principle of confirmation theory: if an observation O is more likely to occur under hypothesis HI than under hypothesis H2, then O supports accepting HI over H2. The strong version of the argument construes the critical term 'support' as connoting 'epistemically warranted'. On this construction, the fine-tuning argument purports to establish that God exists and hence counts, so to speak, as a proof of God's existence.1 The weak version construes 'support' as connoting 'counts as evidence in favor of. On this construction, the argument purports merely to show that the appearance of fine-tuning in the universe is a reason for preferring the hypothesis that God exists to the hypothesis that God does not exist. I argue that only the weaker version succeeds.