God, Fine-Tuning, and the Problem of Old Evidence

The fundamental constants that are involved in the laws of physics which describe our universe are finely tuned for life, in the sense that if some of the constants had slightly different values life could not exist. Some people hold that this provides evidence for the existence of God. I will present a probabilistic version of this fine-tuning argument which is stronger than all other versions in the literature. Nevertheless, I will show that one can have reasonable opinions such that the fine-tuning argument doesn’t lead to an increase in one’s probability for the existence of God. 1 The fine-tuning argument 2 Objective versus subjective probability 3 Observational selection effects 4 The problem of old evidence 5 Against the fine-tuning argument 6 Many universes 1 The fine-tuning argument This article is about the fine-tuning argument for the existence of God, which runs roughly as follows: Premise 1: The fundamental constants that are involved in the laws of physics which describe our universe (such as the masses of the fundamental particles and the strength ratios between the fundamental forces) are finely tuned for life, in the sense that if some of the constants had values outside some narrow range then life could not exist. (I will call this ‘the fine-tuning evidence’.) Lemma: It would be very unlikely for the universe to have life-permitting fundamental constants by chance. (This follows from Premise 1.) Premise 2: If God created the universe, we would expect it to be lifepermitting. Premise 3: The universe is life-permitting. The Author (2006). Published by Oxford University Press on behalf of British Society for the Philosophy of Science. All rights reserved. doi:10.1093/bjps/axl008 For Permissions, please email: [email protected] Advance Access published on May 16, 2006 Conclusion: Thus, given the fine-tuning evidence, the fact that the universe is life-permitting provides evidence for the existence of God. (This follows from the Lemma and Premises 2 and 3.) This article has two main theses. I will argue that the specific version of the fine-tuning argument I will present below is stronger than all other versions in the literature. I will show that the fine-tuning argument is best presented using a subjectivist interpretation of probability; objections to the fine-tuning argument which rely on a frequency-based objective or logical interpretation of probability are flawed. Nevertheless, I am not a proponent of the fine-tuning argument. My second thesis is that one can have reasonable opinions such that the fine-tuning argument doesn’t lead to an increase in one’s probability for the existence of God. This doesn’t count as a full-scale refutation of the fine-tuning argument, since I admit that one can have reasonable opinions such that the argument does lead to an increase in one’s probability for the existence of God. But I believe that no stronger reply to the fine-tuning argument is successful. The fine-tuning argument is generally not taken, even by its proponents, to provide a definitive proof of the existence of God. (In contrast, proponents of the ontological and cosmological arguments generally present their arguments as purely deductive.) As a result, the fine-tuning argument is well-suited to be represented in a probabilistic framework. So let’s look at the argument as formulated using probability theory. The basic version of the probabilistic fine-tuning argument I will be discussing in this article is as follows. (The fine-tuning argument is presented in this sort of way by, for example, Swinburne ([1990], p. 155; [2004], p. 189), Le Poidevin ([1996], pp. 47–8), Collins ([1999], p. 57), Holder ([2002], pp. 298–9), and Manson ([2003], p. 7). There are other versions of the fine-tuning argument in the literature, but I will be focusing on this one.) Let L be the proposition that the universe is life-permitting, and let G be the proposition that God exists. According to proponents of the fine-tuning argument, L provides epistemic support for G. A standard way of understanding the claim that L provides epistemic support for G is to say that learning that L increases one’s probability for G: P(G jL) > P(G). Proponents 1 If one prefers, the proposition G can be taken to include the possibility that some supernatural designer exists, without that designer having all the attributes we would attribute to God. I mention this possibility because some proponents of intelligent design are at pains to maintain that they are not arguing for the existence of God, but just for the existence of a designer. In the case of the fine-tuning argument, that designer would be a designer of the universe, so would presumably have to at least be supernatural. 406 Bradley Monton