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Tractatus LogicoPhilosophicus by Ludwig Wittgenstein + comments by Maarten Maartensz 
What is true is that the value of a truthfunction of p is a function of the value of p  and whether the same applies to meaning or sense is doubtful. A more precise sense in which "Negation, logical addition, logical multiplication, etc. etc. are operations. (Negation reverses the sense of a proposition.)" can be taken from my last remark, with a formal statement of binary Boolean Algebra.
