Note 1 — PART I The Special Theory of Relativity
Robert W. (Robert William) Lawson translation
It follows that a natural object is associated also with a straight line. Three points A, B and C on a rigid body thus lie in a straight line when the points A and C being given, B is chosen such that the sum of the distances AB and BC is as short as possible. This incomplete suggestion will suffice for the present purpose. ↑