Hence, if with reference to two consecutive dots in our block of film a mathematical quantity can be found which will not change no matter how we changes our axes of coordinates, that quantity must be an expression of the true law of motion of the fly between the two points of the paper and the two times represented by these two dots. Einstein has worked out such a quantity remaining constant for all changes of coordinates of the four dimensional world of space-time. In passing we may notice a feature of Einstein's world of space-time which we shall doubtless find it difficult to conceive, namely, that there is no essential difference between a time and a distance in space. Since one set of coordinates is as good as another, we can transform time into space and space into time according as we choose our axes. For example if we change $OX$, $OT$, the axes of $x$ and time in Fig. 2, into $OX'$, $OT'$ by a simple rotation, the new time represented by $OT'$ consists partly of $OA$ in the old time and partly of $OB in the old $x$ direction. Referring to our block of movie film again, it means that although I might separate the block into space and time by slicing it into the original pictures, I can just as readily slice it in any direction I choose and still get individual pictures representing the motion of the fly but with, of course, new time and space. So whilst I may be believe that a liner has travelled 3,000 miles in 4 days, an observer on a star who knows nothing of my particular axes in space-time may say, with equal truth, that it went 2,000 miles in 7 days. Thus, time and space are not two separate identities in Einstein's view; there only exists a world of four dimensions which we can split up into time and space as we choose.