The great question is, What is it all about? To this question some give one answer and some another; but none, to the writer's knowledge, give so clear an answer as Einstein himself,* and even he answersit by implication rather than directly. Still the implication of his definitions of the Special, or Restricted, and of the General principles is so plain that there is no mistaking it. His definition of the Restricted Principle, which need not be given here, as it is fully dealt with in the following pages, is a compendium of the special theory and it is easily generalized. His definition of the General Principle simply repeats the definition of the Restricted Prin ciple in wider terms, and he makes it quite clear thatelativity as a whole is Me wary of Me stato mom ofgonemlfi/zysz'oal laws informs common to all oosem/ers) It is something of a puzzle why other writers of authority have not given this fact a more prominent place and stated it plainly and explicitly. It may have been because it seemed so obvious as not to require emphasis, but to the writer's mind the greater part of the mystery which has sur rounded the subject has arisen through failure to grasp it. It was certainly so in his own case.