is then, according to Minkowski, of the nature of a time. Such intervals represent elements of possible paths of motion, the velocity being less than that of light.[9] In this case the
-axis may be drawn in the direction of
by suitably choosing the state of motion of the inertial system. If
lies outside of the "light-cone" then
is of the nature of a space; in this case, by properly choosing the inertial system,