Even if the constants which economists wish to determine were less numerous, and the method of experiment more accessible, we should still be faced with the fact that the constants themselves are different at different times. The gravitation constant is the same always. But the economic constants — these elasticities of demand and supply — depending, as they do, upon human consciousness, are liable to vary. The constitution of the atom, as it were, and not merely its position, changes under the influence of environment.

Part I : Welfare and the National Dividend, Ch. 1 : Welfare and Economic Welfare, § 4 - The Economics of Welfare (1920)