Willard Van Orman Quine
If pressed to supplement Tweedledee's ostensive definition of logic with a discursive definition of the same subject, I would say that logic is the systematic study of the logical truths. Pressed further, I would say that a sentence is logically true if all sentences with its grammatical structure are true. Pressed further still, I would say to read this book.
It is obvious that truth in general depends on both language and extra-linguistic fact. The statement 'Brutus killed Caesar' would be false if the world had been different in certain ways, but it would also be false if the word 'killed' happened rather to have the sense of 'begat'. Thus one is tempted to suppose in general that the truth of a statement is somehow analysable into a linguistic component and a factual component. Given this supposition it next seems reasonable that in some statements the factual component should be null and that these are the analytic statements. But for all its a priori reasonableness, a boundary between the analytic and synthetic statements simply has not been drawn. That there is such a distinction to be drawn at all is an unempirical dogma of empiricists, a metaphysical article of faith.
Just as the introduction of the irrational numbers ... is a convenient myth which simplifies the laws of arithmetic ... so physical objects are postulated entities which round out and simplify our account of the flux of existence... The conceptional scheme of physical objects is likewise a convenient myth, simpler than the literal truth and yet containing that literal truth as a scattered part.