Philosophy Lexicon of Arguments

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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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I 174f
necessary / modal / Geach: it is even necessary necessary, that no one who is a brother is not male. Because if it were necessary only contingently, it would still be a possible option to be a brother that is not a male, even if no real opportunity! (> NNa, MMa) - if Descartes had said that necessarily God would have to create the necessity that a brother is male he would have talked big nonsense.

Gea I
P.T. Geach
Logic Matters Oxford 1972


> Counter arguments against Geach
> Counter arguments in relation to Modal Logic



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Ed. Martin Schulz, access date 2017-05-25