Philosophy Lexicon of Arguments

Modal logic: the modal logic is an extension of classical logic to systems in which possibility and necessity can also be expressed. Different approaches use operators to express "necessary" and "possible", which, depending on the placement within formulas, can let claims of different strengths win. E.g. there is an object which necessarily has the property F/it is necessary that there is an object with the property F. The introduction of possible worlds makes quantification possible for expressing possibility (There is at least one world in which ...) and necessity (For all worlds is valid ...). See also operators, quantifier, completion, range, possible worlds.
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Books on Amazon
I 144
quantified modal logic / Stalnaker: arises not simply from the joining of modal predicate logic and extensional quantifier theory. - Problem: the increase in expressiveness allows Leibniz’s Law and the existential generalization appear doubtful - problems: 1st Status of sentences - 2nd Relation between domains of individuals.

Sta I
R. Stalnaker
Ways a World may be Oxford New York 2003

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Ed. Martin Schulz, access date 2017-05-23