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 scrupu- 
lous attention to elementary distinctions.