Man, being the servant and interpreter of Nature, can do and understand so much and so much only as he has observed in fact or thought of the course of nature; beyond this he neither knows anything nor can do anything.
We must see whether the same clock with weights will go faster at the top of a mountain or at the bottom of a mine; it is probable, if the pull of the weights decreases on the mountain and increases in the mine, that the earth has real attraction.