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|II 64 f
Counterfactual/Frege: "If iron was lighter than water, it would float on water."
Here, we have the two notions that iron is not lighter, and that something that is lighter floats on water. The subordinate clause again expresses the one notion and a part of the other notion.
E.g. "After Schleswig-Holstein was torn from Denmark, Prussia and Austria quarreled."
Here, the subordinate clause expresses not only one notion, but also part of the other. Therefore, it cannot be replaced generally by another of the same truth value.
Cases in which this simply does not work:
1) That the subordinate clause means no truth value because it only expresses part of a thought.
2) That he does mean a truth value, but does not limit itself to it, because its sense still comprises a portion of the other thought and not only one thought.
The first case occurs
a) In the case of the odd meaning of the words
b) If a part of the sentence only vaguely suggests, instead of being a proper name.
Die Grundlagen der Arithmetik Stuttgart 1987
Funktion, Begriff, Bedeutung Göttingen 1994
Logische Untersuchungen Göttingen 1993