|Private language: according to L. Wittgenstein a private language, i.e. a language which an individual develops only for himself and uses to express his feelings, is not possible. (See L. Wittgenstein, Philosophical Investigations, 1953, § 243, 258). You cannot give yourself instructions. You could not even understand the language. See also rule following, Kripke's Wittgenstein, priviledged access, Wittgenstein's beetle.|
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VsLewis: E.g. Assuming a person who lives isolated throughout his life could spontaneously begin to use a language one day due to his brilliant talent, e.g. to write a diary.
Private Language: This would be a random private language, it would not be subject to the verdict of Wittgenstein. And here no convention would be involved.
LewisVs: Even the isolated living person adheres to a certain regularity. He also knows that he adhered to this regularity in the past and has an interest to behave equally all the time.
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