The argument from fine-tuning holds that it is remarkable that we have a universe fit for life. Given all of the possible ways that the universe might have been, it was vastly improbable that it would turn out to be habitable. Miraculously, though, it did; this, according to the argument from fine-tuning, points towards an intelligent designer creating with life in mind.

One of the most powerful responses to this argument appeals to the many-worlds hypothesis. The many-worlds hypothesis holds that our universe is not the only universe, that ours is just one of many worlds. In some forms, the many worlds hypothesis holds that all possible universes exist, that everything that is possible is actual.

Modal Logic and the Many Worlds Hypothesis

Recently, some philosophers have started taking the possibility of multiple universes more seriously. David Lewis pioneered an approach to modal logic known as possible world modality, according to which statements such as “It is possible that p” are to be understood to be shorthand for “There is a possible world in which p”, and statements such as “It is necessary that p” are to be understood to be shorthand for “In all possible worlds p”. This theory of modality is now widely used, although it is generally regarded with suspicion.

A consequence of possible world modality is that either the many worlds hypothesis is true or most statements about possibility and necessity are false. If “It was possible for John Kerry to win the Presidential race” is shorthand for “There is a possible world in which John Kerry won the Predidential race”, and our universe is the only universe, then “It was possible for John Kerry to win the Presidential race” is false; in the only possible world that there is, our own, Kerry lost.

This thought led David Lewis to affirm modal realism, the view that all possible worlds (including the actual world) are equally real. The many worlds hypothesis, Lewis argued, underpins our views about possibility and necessity, and is therefore too philosophically useful to be rejected.

Implications for the Argument from Fine-Tuning

If the many worlds hypothesis is true, then it does indeed undermine the argument from fine-tuning. If there are many universes, then that vastly increases the odds that one of them will be capable of sustaining life, and so makes an appeal to chance a more plausible explanation of our existence that it would be if there were only one universe.

The key issue, then, is whether the many worlds hypothesis is sufficiently well supported to be anything more than ad hoc speculation? We must weigh up the relative plausibilities of the theory that God exists and the theory that there are many parallel universes.