40 NATURE OF CAPITAL AND INCOME [Cuae. II

utility, that equal increments of wealth have decreasing
increments of utility, would be a contradiction in terms.
To plead in extenuation of such confusions the fact that
popular usage is guilty of them, is like trying to justify in
the science of physics a jumbling together of the concepts
of mass and density, or of velocity and acceleration, or of
force and energy, on the ground that the ordinary man does
not distinguish between them. The proper method of
avoiding large errors in any science is to avoid small ones
at the outset. This can be accomplished only by scrupulous
 attention to elementary distinctions.