This chapter has two combined aims. First, I point out that the standard fine-tuning argument for the multiverse, as discussed in the previous two chapters, differs crucially from paradigmatic instances of anthropic reasoning such as, notably, Dicke's and Carter's accounts of large number coincidences between large numbers in cosmology. The key difference is that the standard fine-tuning argument for the multiverse treats the existence of forms of life as calling for a response and suggests to infer the existence of a multiverse as the best such response. Anthropic reasoning of the type championed by Dicke and Carter, in contrast, assumes the existence of forms of life as background knowledge when assessing whether the large number coincidences are to be expected, given the competing theories. The second aim of this chapter is to propose a new fine-tuning argument for the multiverse, which – unlike the standard one – is structurally similar to Dicke's and Carter's accounts of large number coincidences. The new argument turns out to have the virtue of being immune to the inverse gambler's fallacy charge.

The most-discussed objection against this argument is that it commits the inverse gambler's fallacy, originally identified by Ian Hacking. This fallacy consists in inferring from an event with a remarkable outcome that there have likely been many more events of the same type in the past, most with less remarkable outcomes. I discuss several suggested analogs to the problem of the fine-tuned parameters. Ultimately, as I argue, established standards of rationality may just not allow one to decide whether the standard fine-tuning argument for the multiverse commits the inverse gambler’s fallacy or not. Some of the considerations in this chapter, as explained along the way, are relevant to the debate about the Fermi paradox.

One of the most popular responses to the fine-tuning considerations is to suggest that parameters might have been set to life-friendly values by some cosmic designer or God. I discuss two important objections against this response: first, that we cannot infer anything from life-friendly parameters because we could not possibly have found ourselves in a lifeless universe (the anthropic objection) and, second, that there is no specific conception of the designer according to which the inference to a designer would be plausible: anthropomorphic conceptions of the designer are no longer credible; and on non-anthropomorphic conceptions, the designer’s preferences are inscrutable. I reject the first objection and endorse the second.

According to many physicists, several aspects of the laws of nature, the constants, and the cosmic boundary conditions are fine-tuned for life: had they been slightly different, life would not have existed. Here I review the claimed instances of fine-tuning and some of the criticism that has been levelled against the fine-tuning considerations. I also discuss in which sense, if any, fine-tuned parameters may qualify as improbable. Finally, I review the naturalness criterion of theory choice and discuss how violations of naturalness may be regarded as relevant to the discussion about fine-tuning for life.