The Special Theory of Relativity leads to the following conclusions : (i) The three dimensions of space and one of time constitute an isotropic fourfold, in which there is no unique time-direction, just as there is no unique space direction. (ii) Specifying the velocity of a body is equivalent to specifying its “ time-direction”, that is, the direction of its path through space-time (its “ world-line ”). (iii) If the units of distance and time are taken to correspond with each other, so that the velocity of light is unity, then the lines in our diagram ( L'OL , M'OM , and lines parallel to these) which represent the paths of light pulses (Robb's “ optical lines ”) are at right angles for light going in opposite directions, and in this case the time direction ( T'OT ) of a particle and its corresponding space-direction ( X'OX ) are equally inclined to the optical line ( L'OL ) between them, so that setting a particle in motion involves “ rotating ” its time-direction and its space-direction through equal angles (T 1 OT 2 , X 1 OX 2 towards the optical line which goes in the direction of motion of the particle.