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Tractatus Logico-Philosophicus by Ludwig Wittgenstein + comments  by Maarten Maartensz   



3.311 An expression presupposes the forms of all the propositions in which it can occur. It is the common characteristic mark of a class of propositions.

Again far too obscure. What W. calls "form" seems to be what I would call "grammatical category" (like "noun", "adjective", "name", "proposition").