Metaphysics The
ontological status of possible worlds has provoked intense debate.
David Lewis famously advocated for a position known as
modal realism, which holds that possible worlds are real, concrete places which exist in the exact same sense that the actual world exists. On Lewis's account, the actual world is special only in that we live there. This doctrine is called
the indexicality of actuality since it can be understood as claiming that the term "actual" is an
indexical, like "now" and "here". Lewis gave a variety of arguments for this position. He argued that just as the reality of atoms is demonstrated by their explanatory power in physics, so too are possible worlds justified by their explanatory power in philosophy. He also argued that possible worlds must be real because they are simply "ways things could have been" and nobody doubts that such things exist. Finally, he argued that they could not be reduced to more "ontologically respectable" entities such as maximally consistent sets of propositions without rendering theories of modality circular. (He referred to these theories as "ersatz modal realism" which try to get the benefits of possible worlds semantics "on the cheap"). Modal realism is controversial.
W.V. Quine rejected it as "metaphysically extravagant". Stalnaker responded to Lewis's arguments by pointing out that a way things could have been is not itself a world, but rather a property that such a world can have. Since properties can exist without them applying to any existing objects, there's no reason to conclude that other worlds like ours exist. Another of Stalnaker's arguments attacks Lewis's
indexicality theory of actuality. Stalnaker argues that even if the English word "actual" is an indexical, that doesn't mean that other worlds exist. For comparison, one can use the indexical "I" without believing that other people actually exist. Some philosophers instead endorse the view of possible worlds as maximally consistent sets of propositions or descriptions, while others such as
Saul Kripke treat them as purely
formal (i.e. mathematical) devices.
Explicating necessity and possibility At least since Aristotle, philosophers have been greatly concerned with the logical statuses of propositions, e.g. necessity, contingency, and impossibility. In the twentieth century, possible worlds have been used to explicate these notions. In modal logic, a proposition is understood in terms of the
worlds in which it is true and
worlds in which it is false. Thus, equivalences like the following have been proposed: •
True propositions are those that are
true in the actual world (for example: "
Archduke Franz Ferdinand was assassinated in 1914"). •
False propositions are those that are
false in the actual world (for example: "Archduke Franz Ferdinand was assassinated in 2014"). •
Possible propositions are those that are
true in at least one possible world (for example: "Archduke Franz Ferdinand survived the assassination attempt against him in 1914"). This includes propositions which are necessarily true, in the sense below. •
Impossible propositions (or
necessarily false propositions) are those that are
true in no possible world (for example: "Melissa and Toby are taller than each other at the same time"). •
Necessarily true propositions (often simply called
necessary propositions) are those that are
true in all possible worlds (for example: "2 + 2 = 4"; "all bachelors are unmarried"). •
Contingent propositions are those that are
true in some possible worlds and false in others (for example: "Archduke Franz Ferdinand was assassinated in 1914" is
contingently true and "Archduke Franz Ferdinand survived the assassination attempt against him in 1914" is
contingently false).
Other uses Possible worlds play a central role in many other debates in philosophy. These include debates about the
Zombie Argument,
physicalism, and
supervenience in the
philosophy of mind. Many debates in the
philosophy of religion have been impacted by the use of possible worlds, such as debates about the
ontological argument, the
problem of evil, and
God's foreknowledge. ==History of the concept==