"Moreover, growing uncertainty surrounded even the one too which the academic philosophers felt they could trust: logic. Two centuries before, Kant had asserted in his Logik (1800): ‘There are but few sciences that can come into a permanent state, which admits of no further alteration. To these belong Logic … We do not require any further discoveries in Logic, since it contains merely the form of thought.’ As late as 1939, a British philosopher asserted: ‘Dictators may be powerful today, but they cannot alter the laws of logic, nor indeed can even God do so.’ Thirteen years later the American philosopher Willard Quine calmly accepted that the definition of logic was undergoing fundamental change: ‘What difference is there in principle between such a shift and the shift whereby Kepler succeeded Ptolemy, or Einstein Newton, or Darwin Aristotle?’ In the decades that followed, many rival systems to classical logic emerged: Bochvar’s many-valued logic, new systems by Birkhoff and Destouches-Février and Reichenbach, minimal logic, deontic logics, tense logics. It became possible to speak of empirical proof or disproof of logic."