# proof of validity of tautology in first order logic

Every first-order logic formula which has a tautological shape in propositional logic is a valid formula. Will it be possible to give a formal proof for the above ? Thanks and Regards.

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 Yes it is indeed possible to give a formal proof. And it is proved in many textbooks. Which have you looked at? – Peter Smith Nov 6 '12 at 17:42 i hav not looked at any textbooks as yet.can you please give any reference.so far i have been sticking to my notes only. – heboy Nov 6 '12 at 17:47 I know not the tastes of others, but I learned my set theory from Bourbaki.Hope you like it. – awllower Nov 6 '12 at 17:56 You might want to take a look at Enderton, H. B. (2002) A Mathematical Introduction to Logic, Harcourt/Academic Press, ISBN 0-12-238452-0. Note that in first order logic, such tautologies (or formulas which have "tautological shape in propositional logic) are a proper subset of logically valid formulas, which I presume you know. – amWhy Nov 6 '12 at 18:05