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Subset / Member/Element / Frege: must always be distinguished. FregeVsSchröder / FregeVsareas calculus/"Gebietekalkül": - zero should not be included as an element in each class. - Otherwise, they would be dependent on the particular variety. - Once it were nothing, once it were something. (E.g. negation of a) - solution: zero as a subset (empty set).
Die Grundlagen der Arithmetik Stuttgart 1987
Funktion, Begriff, Bedeutung Göttingen 1994
Logische Untersuchungen Göttingen 1993