A class of facts about the physical universe — that the cosmological constant, the strength of the strong nuclear force, the mass ratio of the proton and the neutron, and several other quantities take values that, had they differed by relatively small fractions, would have produced a universe in which atoms could not form or stars could not burn — has acquired, in the philosophical literature of the past forty years, the name 'fine-tuning.' The word is unfortunate, because it carries a suggestion of intentional adjustment that the empirical fact alone does not warrant. The empirical fact, stripped of suggestion, is that the values are what they are, that we can compute what would have happened had they been different, and that, on those computations, life as we know it would not have arisen.
From this fact, several distinct philosophical proposals have been advanced. The 'design' proposal infers an intentional adjuster from the apparent narrowness of the life-permitting range. The 'multiverse' proposal supposes that physical reality includes many universes with varied parameter values, and that we naturally find ourselves in one of the rare life-permitting ones. The 'necessity' proposal, less explored, supposes that the values are not freely variable in the first place, and that what looks like fine-tuning is, on a deeper theory, a logical consequence we have not yet derived. Each proposal has merits and difficulties; none has secured the consensus its strongest defenders sometimes claim for it.
A fourth response, less ambitious and possibly more honest, declines to read any of the proposals into the data and observes that the empirical fact establishes only the conditional: if the values had been other than they are, life of our kind would not exist. The conditional is true and uncontested; it does not, on its own, license inference to a designer, a multiverse, or a necessity. The leap from the conditional to any of the explanatory frames requires premises that the conditional itself does not supply.
What the case illustrates is something about how the language of philosophical inference can outrun the data it is supposed to handle. 'Fine-tuning' sounds like a problem demanding a solution; the empirical fact behind it is a problem only on the assumption that the values had some prior probability of being different — and that assumption is itself something philosophy of physics has not yet provided a clear account of. Until it does, the apparent urgency of the choice among the proposals may be, in some measure, an artefact of the unfortunate name.
The primary purpose of the passage is to: