Hello all I have been at ease with spacetime diagrams of the usual sort where the y and z dimensions are suppressed until I came across this passage from the following book. Problems of Space and Time. A reader edited and with an introduction by J.J.C.Smart. Page 293 of the chapter entitled The Four Dimensional World by Moritz Schlick. Reprinted from Chapter 7 of The Philosophy of Nature 1949. -----The world-lines describe the motion of particles; but they must not be mistaken for the tracks of these particles. One may not, for example, that a point traverses its world-line; or that the three dimensional section which represents the momentary state of the actual present, wanders along the time axis through the four dimensional world. For a wandering of this kind would have to take place in time; and time is already represented within the model and cannot be introduced again from outside.------ My understanding was that a point on a world-line represented the position of a particle in space at a certain time, a sort of plot of position, or track, against time. So I am either mistaken or I fail to understand the significance of the above passage, or both. Can anyone expand on this. Matheinste.