Suppose a fly is crawling over this sheet of paper and let us make a movie record of it. If we cut up the strip of movie film into the individual pictures and cement them together one above another in their proper order, we shall build up a solid block of film which will be a model of our simplified world of space-time and in which there will be a series of dots representing the motion of the fly over the paper. Just as I can state the exact position of an object in my room by defining its height above the floor, its distance from the north wall and its distance from the east wall, so we can reduce the positions of the dots to figures for use in calculations by measuring their distances from the three faces intersecting in the lines $OX$, $OY$, and $OT$, where $OXAYTBCD$ represents the block of film. The mathematician would call the three lines $OX$, $OY$, $OT$ the coordinate axes. Measuring all the dots in this way we shall obtain the motion of the fly relative to the coordinate axes $OX$, $OY$, $OT$. If we add a block $OTDYEFGH$ of plain film we can use $EX$, $EH$, $EF$ as coordinate axes and again obtain the motion of the fly relative to these new axes; or we can add block after block so as to keep the axes moving. We can conceive of other changes of axes. The operator making the movie record might have taken the fly for the hero of the piece and moved the camera about so as to keep the fly more or less central in the picture; or he might, by turning the handle first fast and then slow and by moving the camera, have made the fly appear to be doing stunts. Moving the camera would change the axes of $x$ and $y$, and turning the handle at different speeds would change the axis of time. Again, we might change the axes by pushing the block out of shape or by distorting it into a state of strain. Whatever change of axes we make, any dot in the block of film will signify a coincidence of the fly with a certain point of the paper at a certain time, and the series of dots will, in every case, be a representation of the motion of the fly. Maybe the representation will be a distorted one, but who is to say which is the absolutely undistorted representation? The principle of relativity which we laid down before says that no one set of coordinates will give the absolute motion of the fly, so that one set is as good as another. The principle that all motion is relative means, therefore, that no matter how we change our coordinates of space-time, the laws of motion which we deduce must be the same for all changes. To use an analogy, the sculptured head of Shakespeare on my table may appear to have hollow cheeks when I admit light from the east window only, or to have sunken eyes with light from the skylight in the roof, but the true shape of the head remains the same in all lights.