The traditional logic, however, cannot treat the matter so lightly. For the plasticity of words may always engender a conflict between the old meaning and the new, between the scientific use of terms and the traditional conventions about their use. And this can always be represented as a defiance of the ‘laws of thought’. For if the meaning of ‘_A_’ may be altered by the growth of knowledge, it will no longer be true that everything once called ‘_A_’ is truly _A_, nor that what was once incompatible with _A_ will continue to be so for all time. Hence it is no longer necessarily true that ‘_A_ is _A_’, and that _A_ cannot both be and not be _B_. It may be both in different senses, and in what sense ‘_A_’ and ‘_B_’ should be taken may be precisely the point at issue. Thus verbal contradiction ceases to be a clear proof of error; it may be only a much-needed warning that our terms have been developing new meanings. Hence, the ‘laws’ of Identity and Contradiction lose their last claims to be regarded as statements of fact, and have to be conceived as ideal postulates of just so much stability of meaning as is requisite for effective understanding.[402] They can be applied to reality only hypothetically, i.e. experimentally, to discover whether in a given situation the natural growth in the meaning of the terms may _rightly_ be treated as irrelevant, and does not vitiate the conclusion which the reasoning forecasts. Now this problem can never be settled _a priori_ by reasoning, but only by subsequent experience. Reasoning may forecast a result which experience fails to confirm; when we discover that comets’ tails are not attracted by the sun but repelled, we do not declare the facts ‘contradictory’, but modify our notion of ‘gravitation’, and conceive it as inferior to ‘light pressure’ in its effects upon particles of a certain minuteness. It follows that no merely logical scrutiny of the terms of an argument can ever settle a scientific question. If a ‘contradiction’ is real, it means either a difference of opinion between those who make the incompatible assertions, or, in the case of a real ‘self-contradiction’, the uttering of ‘nonsense’ and a failure to propound a meaning at all. But even the most glaring ‘contradictions’ may only be apparent, i.e. verbal: when we inquire into their actual meaning we may find that they refer to a context in which its terms are perfectly compatible. Thus the existence of a ‘round square’ may be predicated of London, and a ‘triangle’s’ angles may equal or may exceed two right angles, according as it belongs to Euclid’s geometry or to Riemann’s.