We come now to the question: what is a priori certain or necessary, respectively in geometry (doctrine of space) or its foundations? Formerly we thought everything; nowadays we think nothing. Already the distance-concept is logically arbitrary; there need be no things that correspond to it, even approximately.
Albert Einstein (1879 - 1955)
Source: "Space-Time." Encyclopaedia Britannica, 14th ed.
Contributed by: Zaady