|Equivalence: Relation between sentences. It exists if both sides have the same truth value, so that they are both true or both false.|
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|Equivalence||SL I 6
Equivalences/modal logic/Stuhlmann-Laeisz: relationship between operators - E.g. necessary: not possible that ~ a - offered: if not allowed that a ~ - believed: if not thought possible that ~ a. - Will always be the case: if not once the case that ~ a.
Freges Logische Untersuchungen Darmstadt 1995