How Do You Talk About What Might Have Been? Invent an Infinity of Worlds
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Philosophers analyse claims about necessity, possibility and what would have happened by treating them as claims about complete alternative ways things could have been.
The device being used
Statements about what must be true, what could be true and what would have happened resist ordinary analysis, because they are not simply about how things are. The standard move is to treat each as a claim quantifying over complete alternative situations. Something is necessary if it holds in all of them, possible if it holds in at least one, and impossible if it holds in none. A claim about what would have happened had things gone otherwise is a claim about what holds in those alternatives most similar to how things actually are.
What the device buys
Recasting the problem this way makes several things tractable:
- •Necessity and possibility become ordinary quantifiers
- •The logic of those notions can then be formalised and proved about
- •Statements about what would have been get truth conditions
- •Relations between different alternatives can be varied and studied
- •Different assumptions about those relations give different logics
- •Meaning itself can be modelled as a function across alternatives
The question of what they are
The device works and what its elements actually are is fiercely contested. One position holds that they are concrete, that alternative situations exist in exactly the sense our own does, that their inhabitants are as real as we are, and that actual simply means here. David Lewis argued that at length and accepted the consequences. Most philosophers find that too much to swallow and take the alternatives to be abstract instead, as maximal sets of propositions, or ways things could have been, or something constructed out of the actual world. The dispute is about the price of a tool nobody wants to give up.
The problem of identity across them
A specific puzzle troubles the framework and it is the one that generates most of the literature. Saying that somebody could have chosen differently seems to require that a person in some alternative situation is the same person as the one here, and what makes them the same is not obvious, since they differ in at least one respect by construction. One answer says the same individual genuinely exists in many alternatives. Another says each alternative contains a counterpart, similar but not identical, and that claims about what somebody could have done are claims about their counterparts. Both answers have uncomfortable consequences.
Where the idea reaches
The framework escaped philosophy and is now used in several technical fields without any metaphysical commitment. Computer science uses closely related structures to verify that a system can never reach a bad state, checking every reachable configuration rather than every alternative universe. Linguistics uses the framework to analyse conditional sentences and words expressing degrees of certainty. Economics and decision theory use states of the world in essentially the same role. In every case the useful part is the structure, and the question of what the alternatives are remains a philosophical one that the applications simply decline to answer.
The takeaway
Treating necessity as truth in all complete alternative situations, and possibility as truth in at least one, turns hard claims about what must or could be into ordinary quantification. Claims about what would have happened become claims about the most similar alternatives. What those alternatives actually are is contested, with concrete and abstract accounts both defended.