|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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Modal logic / possibility / necessity / Chisholm / Sauer: from M follows NM and vice versa: what is possible is necessarily possible - "There is a world, so that p each possible world is such that there is a world, so that p" - (s) no world excludes other worlds? - Suaer: NM is applicable when possibility is limited only by consistency - consistency is independend of which non-logical propositions are true.
Die erste Person Frankfurt 1992
Roderick M. Chisholm
Erkenntnistheorie Graz 2004