The purely relative aspect of the matter is further brought out if we consider a single example both backwards and forwards. Systems $S$ and $S'$ are in relative motion. An object in $S$ which to an observer in $S$ is $L$ units long, is shorter for an observer in $S'$--shorter by an amount indicated through the "correction factor" $K$. Now if we have, in the first instance, made the objectionable statement that objects are shorter in system $S'$ than they are in $S$, it will be quite natural for us to infer from this that objects in $S$ must be longer than those in $S'$; and from this to assert that when the observer in $S$ measures objects lying in $S'$, he gets for them greater lengths than does the home observer in $S'$. But if we have, in the first instance, avoided the objectionable statement referred to, we shall be much better able to realize that the whole business is quite reciprocal; that the phenomena are symmetric with respect to the two systems, to the extent that we can interchange the systems in any of our statements without modifying the statements in any other way. Objects in $S$ appear shorter and times in $S$ appear longer to the external "moving" observer in $S'$ than they do to the domestic observer in $S$. Exactly in the same way, objects in $S'$ appear shorter to observers in the foreign system $S$ than to the home observer in $S'$, who remains at rest with respect to them. I think that when we get the right angle upon this situation, it loses the alleged startling character which has been imposed upon it by many writers. The "apparent size" of the astronomer is an analogy in point. Objects on the moon, by virtue of their great distance, look smaller to observers on the earth than to observers on the moon. Do objects on the earth, on this account, look larger to a moon observer than they do to us? They do not; any suggestion that they do we should receive with appropriate scorn. The variation in size introduced by distance is reciprocal, and this reciprocity does not in the least puzzle us. Why, then, should that introduced by relative motion puzzle us? TIME AND SPACE IN A SINGLE PACKAGE