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Symmetry/Identification/Mentalese/Hintikka: (Assuming that there is a form in Mentalese), then the two sides of symmetry could be distinguished only semantically, but not syntactically.
Jaakko and Merrill B. Hintikka
The Logic of Epistemology and the Epistemology of Logic Dordrecht 1989
J. Hintikka/M. B. Hintikka
Untersuchungen zu Wittgenstein Frankfurt 1996