Indistinguishability

"Supposing, for example, that in carrjing our eye along a log of wood, and a piece of stone^ we find the troublof moving the eye (which, though small, is perceptible) to be neither greater nor less in the one case than in the other, these two efforts will e then be indistinguishable to us ; and if we find many cases of such indistinguishability, we shall want a word to express the idea ; we shall call it equality."