IN NATURE of October 28, p. 572, Dr. Robb pointed out the incorrectness of asserting that the length of a “world-line” is a minimum between any two points of it. He gave an example in which the length was neither a minimum nor a maximum. The object of his letter, no doubt, was to remind some reckless relativists that they should be more careful in their language. But there is the danger that some may suppose that he was dealing with a real weakness in Einstein's theory. To dispel this idea we may recall a few well-known facts.