THE fundamental postulate of the special theory of relativity, and a vital principle of the general theory of relativity, is that there exists a maximum speed of propagation, confining the link between cause and effect to the surface or interior of the light cone. It is important to realize that, particularly in the special theory, this postulate is used essentially not in the mathematical but in the logical sense. That is to say, it is asserted that causal links and links of information outside the light cone do not exist, rather than that such links are more difficult to describe mathematically than links on or within the light cone. This logical assertion is used to the full in the denial of the possibility of clock synchronization.