  Since the relative velocities of matter is small in each collection  of galaxies extending over an astronomical neighbourhood, one can  approximate a “smeared-out” motion of the galaxies and  introduce a substratum or fluid which fills space  and in which the galaxies move like “fundamental   particles”.[83]  Weyl’s postulate says that observers associated with this smeared-out motion constitute a privileged class of observers of the universe.  Since geodesics do not intersect, according to Weyl’s Postulate,  there exist one and only one geodesic which passes through each  spacetime point. Consequently, matter possesses a unique velocity at  any spacetime point. Therefore, the fluid may be regarded as a  perfect fluid; and this is the essential content of Weyl’s  Postulate. 