Long ago I accepted that the speed you move through time is personal, but I've always wanted to really understand it intuitively. I think I'm almost there. Suppose me and my ghost are in the same place initially. Now I take off running in a direction, but my ghost remains behind. Since I'm moving through space faster, my motion through time is slower because the space-time interval is the same for both of us (I hope this much is right). Now my question: Why are the two space-time intervals considered equivalent?