To express roughly what is meant is, I think, simple enough; and everybody will recognise it at once, as a notion which is constantly in people's heads; but I want to dwell upon it at some length, because I know of no place where it is expressly explained and defined, and because, though it seems very simple and fundamental, the task of defining it precisely is by no moans easy and involves some difficulties which I must confess that I do not know how to solve.