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|Counterfactual Conditionals||II 64
Counterfactual Conditional/Frege: expresses two thoughts - E.g. "if iron was lighter than water, it would float on water" - 1) That iron is not lighter - 2) That something that is lighter... - The subordinate clause expresses one thought and a portion of the other. - Therefore it cannot be replaced by another sentence of the same truth value.
Die Grundlagen der Arithmetik Stuttgart 1987
Funktion, Begriff, Bedeutung Göttingen 1994
Logische Untersuchungen Göttingen 1993