Locality of interaction: We assume that the the influence that one particle has on another cannot be transmitted instantaneously (why not?). It does not have to be photon at light speed as in one electron exerting a force on another electron. How does one electron 'know' of the existence of the other electron - some type of photon interation we assume and it cannot travel at infinite speed (why not?). We could think of two masses that are 'aware' of each other, not by photon interation this time but gravity that travels at some speed using some agent. Why cannot that speed be infinite? There must be some simple illogicality that results if that speed were infinite. I know its probably easy but I simply don't know the reason and would love to know. Thanks in advance for non-obfuscating replies.