"What would be the consequences for the theory of truth, asked one worried logician,’… of the adoption of a non-standard system’? Another, observing systems of modal logic, observed: ‘One gets an uneasy feeling as one discerns and studies more of the systems belonging to this family that it is literally a family, and has the power of reproducing and multiplying, proliferating new systems [of logic] without limit.’ In a world in which even the rules of logic shifted and disintegrated, it is not surprising that modern times did not develop in ways the generation of 1920 would have considered ‘logical’."

"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."

"Logician: A cat has four paws. Old Gentleman: My dog had four paws. Logician: Then it's a cat. Old Gentleman: So my dog is a cat? Logician: And the contrary is also true."

"I have expos'd myself to the enmity of all metaphysicians, logicians, mathematicians, and even theologians; and can I wonder at the insults I must suffer?"

"But in this age, logic was a flame that must be frequently starved of fuel."

"Logic is one thing and commonsense another."

"Logic is logic. That's all I say."

"To understand this for sense it is not required that a man should be a geometrician or a logician, but that he should be mad."

"Logic is a feeble reed, friend. "Logic" proved that airplanes can't fly and that H-bombs won't work and that stones don't fall out of the sky. Logic is a way of saying that anything which didn't happen yesterday won't happen tomorrow."

"Logic by definition is that which makes sense: nothing more, nothing less. No miracles. No supernatural hocus-pocus. Everything that takes place, every event, every effect, is logically caused by something, something which preceded it in time, and which provided the physico-chemical causative chain that resulted in the effect. These premises are the foundations of our intellectual existence."

"To find themselves utterly alone at night where company is desirable and expected makes some people fearful; but a case more trying by far to the nerves is to discover some mysterious companionship when intuition, sensation, memory, analogy, testimony, probability, induction — every kind of evidence in the logician's list — have united to persuade consciousness that it is quite in isolation."

"The want of logic annoys. Too much logic bores. Life eludes logic, and everything that logic alone constructs remains artificial and forced."

"Logic is usually understood nowadays as a study of certain formal systems, though in former times there were philosophers who held that the subject matter of logic was the formal rules of human thought."

"The first question we should face is: What is the aim of a physical theory? To this question diverse answers have been made, but all of them may be reduced to two main principles: "A physical theory," certain logicians have replied, "has for its object the explanation of a group of laws experimentally established." "A physical theory," other thinkers have said, "is an abstract system whose aim is to summarize and classify logically a group of experimental laws without claiming to explain these laws... Now these two questions — Does there exist a material reality distinct from sensible appearances? and What is the nature of reality? — do not have their source in experimental method, which is acquainted only with sensible appearances and can discover nothing beyond them. The resolution of these questions transcends the methods used by physics; it is the object of metaphysics. Therefore, if the aim of physical theories is to explain experimental laws, theoretical physics is not an autonomous science; it is subordinate to metaphysics... Now, to make physical theories depend on metaphysics is surely not the way to let them enjoy the privilege of universal consent."