This is our conception of a world, if such were possible, entirely free from the influence of energy. We may conceive of it as an amorphous immaterial something containing "point-events" (a point-event being an instant of time at a point in space--a conception, not a definition). These point-events have a fourfold order and definite relation in this Frame, i.e. they can be specified by four variables or coordinates in reference to some base called a reference system, with respect to which they are forward or backward, right or left, above or below, sooner or later. This shows the World-Frame to be four-dimensional. Thus an aggregate of point-events (or an "event," which implies limited extension in space and limited duration in time) [7] would have what we familiarly describe as length, breadth, height and time. To express these metrical properties most simply we must choose a four-dimensional reference system having a particular form--rectilinear axes (Cartesian coordinates), and a particular motion--uniform and rectilinear, i.e. unaccelerated, and non-rotating with respect to the path of a light ray. We call this an inertial system because Newton's Law of Inertia holds for such a system alone. This system indicates how observers partition the World-Frame into space and time. It restricts observers to uniform rectilinear motion, and observations to bodies and light-pulses in such motion. Thus gravitational and other forces are discounted, and we obtain World-Frame conditions notwithstanding the fact that observers are in the presence of energy. Now the separation between point-events which have a definite relation to each other must be absolute. The separation between two points in a plane is defined by the unique distance between them (the straight line joining them). Between point-events the analogue of this unique distance, which we call the "separation-interval" (to indicate its time-like and space-like nature), is also unique. Its unique and absolute character give it great importance as thereby it is the same for all observers regardless of their reference system. If, in place of the rather cumbersome expression $X-x$ to indicate the difference between the $x$-coordinates of two points, we employ the more compact expression $dx$; if for the benefit of readers who have a little algebra but no analysis we state explicitly that this expression is a single symbol for a single quantity, and has nothing to do with any product of two quantities $d$ and $x$; and if we extend this notation to all our coordinates: then it is clear from previous essays that the distance $S$ between two points in a plane referred to a rectilinear system $OX$, $OY$, is given by the simple equation $S^2 = (dx)^2 + (dy)^2$. Einstein and Minkowski show that the value for the separation interval $\Omega$, the analogue of $S$, referred to an inertial system is given by the equation $$\Omega^2 = (dx)^2 + (dy)^2 + (dz)^2 - (dt)^2\,,$$