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Definition about/"aboutness"/Lewis: a proposition is about an object iff it applies in either or neither of any two worlds that match perfectly in terms of this subject - the other way around: just as well: two worlds fully coincide with respect to an object iff every proposition is true in both worlds or in none about this subject. - E.g. a proposition that differentiates the two, cannot be about something that is the same in both worlds. - There is no non-circular test for "aboutness" that does not refer to the language used. - Therefore, "about" is not a fundamental concept. - Also not "match in relation to".
Die Identität von Körper und Geist Frankfurt 1989
Konventionen Berlin 1975
Philosophical Papers Bd I New York Oxford 1983
Philosophical Papers Bd II New York Oxford 1986
Cl. I. Lewis
Mind and the World Order: Outline of a Theory of Knowledge (Dover Books on Western Philosophy) 1991