the rietdijk putnam argument?
if special relativity, which is of course einstien's theory that all uniform motion is relative, and that there is no absolute and well-defined state of rest, is true, then each individual observer will have their own plane of simultaneity, which contains a unique set of events that constitutes the observer's present moment, right? observers moving at different relative velocities have different planes of simultaneity hence different sets of events that are present. each observer considers their set of present events to be a three dimensional universe but even the slightest movement of the head or offset in distance between observers can cause the three dimensional universes to have differing content. if each three dimensional universe exists, then the existence of multiple three dimensional universes suggests that the universe is four dimensional.