"A language is open
in case there is another language containing all the expressions of the
first and imparting to them their original meaning, which second language
however contains expressions not occourring in the first (as regard
either sounds or meanings) and at least one of these expressions is immediately meaning-related to an
expression of the first language. A non-open language is called closed.
Open language can have added to them new expressions (not synonymous with any of those already present) which are immediately meaning-related to expressions already present, without thereby altering the meaning of expressions already present. Closed language, on the contrary, become disconnected when ther is added a new expression non synonymous with any of those already present"
("The World-Picture and the Conceptual Apparatus" (1934), in K. Ajdukiewicz, The Scientific World-Perspective and other Essays (1931-1963), ed. by J. Giedymin, Reidel, Dordrecht 1978, p. 69).
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