In short, the universe is regarded as a space-time continuum of four dimensions. A "point" in space-time is called an "event"--that which occurs at a specified moment and at a specified place. The distance between two points in space-time is called their "interval." All observers will agree as to the magnitude of any interval, since it is a property of the objective universe; but they will disagree as to its composition in space and time separately. In short, space and time are relative conceptions; their relativity is a necessary consequence of the relativity of motion. The paradox named at the outset is overcome; for the two observers measuring the velocity of the light produced as they passed one another, were using different units of space and time. And hence emerges triumphant the Special Principle of Relativity, which states that the laws of nature are the same for all observers, whether they are in a state of rest or of uniform motion in a straight line. ACCELERATED MOTION Uniform motion in a straight line is however a very special kind of motion. Our experience in ordinary life is of motions that are neither uniform nor in a straight line; both speed and direction of motion are altering. The moving body is then said to undergo "acceleration": which means either that its speed is increasing or diminishing, or that its direction of motion is changing, or both. If we revert to our former supposition of a universe in which there is only a single body in "empty" space, we clearly cannot say whether it has acceleration any more than whether it is moving, there being no outside standard of comparison; and the General Principle of Relativity asserts the invariance of the laws of nature for all states of motion of the observer. In this case, however, a difference might be detected by an observer on the moving body itself. It would be manifested to him as the action of a force; such for instance as we feel when a train in which we are traveling is increasing or reducing speed, or when, without changing speed, it is rounding a corner. The force dies away as soon as the velocity becomes uniform. Thus acceleration reveals itself to us under the guise of action by a force. Force and acceleration go together, and we may either say that the acceleration is due to the force, or the impression of force to the acceleration.