Impossible world

In philosophical logic, the concept of an impossible world (sometimes non-normal world)
is used to model certain
phenomena that cannot be adequately handled using ordinary possible worlds. An
impossible world, "w", is the same sort of thing as a possible world (whatever that may be),
except that it is in some sense "impossible." Depending on the context,
this may mean that some contradictions are true at "w", that the normal laws of logic or of metaphysics fail to
hold at "w", or both.
Argument from ways.
Possible worlds.
Possible worlds are often regarded with suspicion, which is why their proponents have struggled to find arguments in their favor. An often-cited argument is called the argument from ways. It defines possible worlds as "ways how things could have been" and relies for its premises and inferences on assumptions from natural language, for example:
The central step of this argument happens at "(2)" where the plausible "(1)" is interpreted in a way that involves quantification over "ways". Many philosophers, following Willard Van Orman Quine, hold that quantification entails ontological commitments, in this case, a commitment to the existence of possible worlds. Quine himself restricted his method to scientific theories, but others have applied it also to natural language, for example, Amie L. Thomasson in her "easy" approach to ontology. The strength of the "argument from ways" depends on these assumptions and may be challenged by casting doubt on the quantifier-method of ontology or on the reliability of natural language as a guide to ontology.
Impossible worlds.
A similar argument can be used to justify the thesis that there are impossible worlds, for example:
The problem for the defender of possible worlds is that language is ambiguous concerning the meaning of "(a)": does it mean that this is a way how things couldn't be or that this is not a way how things could be. It is open to critics of impossible worlds to assert the latter option, which would invalidate the argument.
Applications.
Non-normal modal logics.
Non-normal worlds were introduced by Saul Kripke in 1965 as a purely technical device to
provide semantics for modal logics weaker than the system K — in particular, modal logics that reject
the rule of necessitation:
Such logics are typically referred to as "non-normal." Under the standard interpretation of modal vocabulary in Kripke semantics, we have formula_2 if and only if in each model, formula_3 holds in all worlds. To construct a model in which formula_3 holds in all worlds but formula_5 does not, we need either to interpret formula_6 in a non-standard manner (that is, we do not just consider the truth of formula_3 in every accessible world), or we reinterpret the condition for being "valid". This latter choice is what Kripke does. We single out a class of worlds as "normal", and we take "validity" to be truth in every normal world in a model. in this way we may construct a model in which formula_3 is true in every normal world, but in which formula_5 is not. We need only ensure that this world (at which formula_5 fails) have an accessible world which is not "normal." Here, formula_3 can fail, and hence, at our original world, formula_5 fails to be necessary, despite being a truth of the logic.
These non-normal worlds are "impossible" in the sense that they are not constrained by what is true according to the logic. From the fact that formula_2, it does not follow that formula_3 holds in a non-normal world.
For more discussion of the interpretation of the language of modal logic in models with worlds, see the entries on modal logic and on Kripke semantics.
Avoiding Curry's paradox.
Curry's paradox is a serious problem for logicians who are interested in developing formal languages that are "semantically closed" (i.e. that can express their own semantics). The paradox relies on the seemingly obvious principle of contraction:
There are ways of using non-normal worlds in a semantical system that invalidate contraction. Moreover, these methods can be given a reasonable philosophical justification by construing non-normal worlds as worlds at which "the laws of logic fail."
Counternecessary statements.
A counternecessary statement is a counterfactual conditional whose antecedent is not merely false, but "necessarily" so (or whose consequent is necessarily true).
For the sake of argument, assume that either (or both) of the following are the case:
Presumably each of these statements is such that if it is true (false), then it is necessarily true (false).
Thus one (or both) of the following is being assumed:
Now consider the following:
This is intuitively false, as one of the fundamental tenets of intuitionism is precisely that the LEM "does not" hold. Suppose this statement is cashed out as:
This holds vacuously, given either (1′) or (2′).
Now suppose impossible worlds are considered in addition to possible ones. It is
compatible with (1′) that there are "impossible" worlds at which intuitionism is true, and with (2′) that there are "impossible" worlds at which the LEM is false. This yields the interpretation:
This does not seem to be the case, for intuitively there are "impossible" worlds at which intuitionism is true and the law of excluded middle does not hold.