"In scholarship it is not perhaps necessity, but prejudice, that is the mother of invention."

"Loyalty to ideas is not a good thing for … anyone."

"The description of the origin of an idea should not be taken for a definition, nor should an account of the mental and physical conditions for becoming aware of a proposition be taken for a proof. … Otherwise we would find it necessary to take account of the phosphorous content of our brain in proving Pythagoras’ theorem, and astronomers would shy away from extending their conclusions to the distant past, for fear of the objection: “You reckon that 2 x 2 = 4 held then; but the idea of number had a development, a history! One can doubt whether it had reached that stage by then. How do you know that this proposition already existed at that point in the past? Might not the creatures living at that time have held the proposition 2 x 2 = 5, from which the proposition 2 x 2 = 4 only evolved through natural selection in the struggle for existence; and might not this in turn, perhaps, be destined to develop further into 2 x 2 = 3?” … What is called the history of concepts is really a history either of our knowledge of concepts or of the meanings of words."