I came to the topic of the title in connection with my logical investigations of the Is-Ought problem in multimodal logics (Schurz 1997). There are infinitely many mathematically possible modal logics. Are they all philosophically serious candidates? Which modal logic the "right" one — does such a question make sense? A similar question can be raised for the infinite variety of propositional logics weaker than classical logics. The Vienna Circle concept of logic was that logic holds merely by form, independently from the facts of the world. Have we lost this concept completely? Is it a matter of arbitrary choice, of mere subjective-practical appropriateness, which logic one chooses? Is Quine right that there is no distinction between analytic and synthetic truth, even if we take "analytic truth" in the narrow sense of "logical truth"? These are the questions which have motivated this paper.