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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 20th 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, physicalismSee also the old text of this article Physicalism/Larry's text. Physicalism is the metaphysical position that everything is physical; that is, that there are no kinds of things other than physical things. Likewise, physicalism about the mental is a positi and supervenienceIn philosophy, supervenience is a well-defined relation between "higher-level" (e. mental) and "lower-level" (e. physical) properties. Informally, a group of properties X supervenes on (alternatively, is supervenient on a group of properties Y exactly whe in the philosophy of mind. Intense debate has also emerged over the ontologicalThis article is about the philosophical meaning of ontology . For the term in computer science, see ontology (computer science). In philosophy, ontology is the most fundamental branch of metaphysics. It is the study of being or existence as well as the ba 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 AdamsRobert Adams (born 1937) is an American photographer most known for being part of the photographic movement, the New Topography''. Adams was born in the highly industrialized town of Orange, New Jersey before relocating to Colorado when he first started o 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 PlantingaAlvin Plantinga (born 15 November, 1932 in Ann Arbor, Michigan) is a contemporary United States philosopher known for his work in metaphysics and in the philosophy of religion. Plantinga won a 'fat scolarship' for Harvard College, but left it in 1951 to s 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.
