Possible worlds

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In contemporary philosophy, possible worlds other than our
own are a routine conceptual device used to frame arguments that hinge on possibility and necessity. These
include evaluating the truth modal claims about what might or might not have
happened, understanding the meaning of counterfactual conditionals, and thought experiments (especially in metaphysics, the
philosophy of mind, and ethics). The conception of a "possible world" employed here is that of a complete specification of a
way the world could be (or could have been). So, for example, if I could choose to blow off work to go to the
movies today, and I could choose not to blow off work to go to the movies today, then that distinguishes two
sets of possible worlds: those worlds in which I did go to the movies, and those in which I did not. Similarly
for any other contingent fact: there are possible worlds in which it rained
today and possible worlds in which it didn’t; possible worlds in which I went to a dance on Friday, possible worlds in which I
stayed in, and possible worlds in which I had never even been born in the first place. Each possible world, then, represents a
different sum total of all the ways that things could have been.

The explicit notion of "possible worlds" is most commonly attributed to the account of Creation in the writings of Gottfried Leibniz, but scholars have also found traces of the idea in the
writings of Averroes and John Duns Scotus. A systematic theory of possible worlds, however, was first introduced in 20^th
century work on the semantics of modal logic, led by Saul Kripke. Here possible worlds are used to provide a semantics for claims about
possibility and necessity: a statement that is possible is said to be true in at least one possible world; a
statement that is necessary is said to be true in all possible worlds, and a statement that is actual
is said to be one which is true in the possible world in which we reside.

From this groundwork, "possible worlds" became a central part of many philosophical arguments in the 1960s and
1970s—including, most famously, the analysis of counterfactual
conditionals in terms of "nearby possible worlds" developed by David Lewis and Robert Stalnaker. On this analysis, when we discuss would would have happened if some
set of conditions were the case, the truth of our claims is determined by what is true in the nearest possible world (or
the set of nearest possible worlds) where the conditions obtain. (A possible world W₁ is said to be near to
another possible world W₂ in respect of R to the degree that the same things happen in W₁ and W₂
in respect of R; the more different what happens in two possible worlds in a certain respect, the "further" they are from one
another in that respect.) For example, statements like "If the scandal hadn’t broke at the last minute, he’d have won the
election" are to be taken as referring to a nearby possible world in which the scandal hadn’t broke and he’d won the
election. If that possible world is the nearest possible world (in the right respects) where the scandal hadn’t broke, then the
statement is true. If there is a nearer possible world (in the right respects) where the scandal hadn’t broke, but he still lost
the election, then that is reason to take the counterfactual as false.

Today, possible worlds play a central role in many debates within philosophy, including especially debates over the Zombie Argument, physicalism and supervenience in the philosophy of mind. Intense debate has also emerged over the ontological status of possible worlds:—provoked especially by David Lewis’s defense of modal realism, the doctrine that talk
about "possible worlds" is best explained in terms of innumerable, really existing physical worlds other than the one we
live in. The question here is: given that modal logic works, and that the possible worlds semantics for modal logic are
correct, what has to be true of the world, and what are the "possible worlds" that we range over in our interpretation?
Lewis argued that what we range over is nothing more or less than real, physical worlds that can be said to exist in
exactly the same way (and to exactly the same degree) as our actual world, but which are distinguished from the actual world by
having no spatial or temporal relationship to what happens in it. (On Lewis’s account, the only special property that the
actual world has is that we are in it.) Others, such as Robert Adams
(Philosophor) and William
Lycan, reject Lewis’s picture as metaphysically extravagant, and suggest in its place an interpretation of "possible worlds"
as consistent, maximally complete sets of descriptions of or propositions about the world, so that a "possible world" is
conceived of as a complete description of a way the world could be—rather than a world which is that
way. (Lewis describes their position, and similar positions such as those advocated by Alvin Plantinga and Peter Forrest, as "ersatz modal realism", arguing that such theories try to get the benefits of
possible worlds semantics for modal logic "on the cheap", but that they ultimately fail to provide an adequate explanation.)
Saul Kripke, in Naming and
Necessity, took explicit issue with Lewis’s use of "possible worlds" semantics, and defended a stipulative
account of possible worlds as purely formal (logical) entities rather than either really existent worlds or as some set
of propositions or descriptions.

The philosophical theory of possible worlds has often been compared to the many-worlds interpretation of quantum mechanics. The main difference between these two theories is roughly, that the many-worlds
interpretation is an attempt to explain how counterfactual physical events can influence what actually happens
(specifically by quantum entanglement), whereas the
Possible Worlds theory is an attempt to explain any counterfactual situation, including those in which the laws of
physics or other large-scale features of the universe differ. In fact, if the many worlds interpretation is true, and
conterfactual events can influence actuality, then the philosophical theory would need to step back and talk about possible
universes of many worlds, so as to account for what would have been the case if the different events had transpired in the
quantum-physical worlds.

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