What we really have to substitute for force is laws of correlation. Events can be collected in groups by their correlations. This is all that is true in the old notion of causality. And this is not a “postulate” or “category”, but an observed fact--lucky, not necessary. As we suggested before, it is these correlations of events that lead to the definition of permanent “things”. There is no essential difference, as regards substantiality, between an electron and a light-ray. Each is really a string of events or of sets of events. In the case of the light-ray, we have no temptation to think otherwise. But in the case of the electron, we think of it as a single persistent entity. There _may_ be such an entity, but we can have no evidence that there is. What we can discover is (_a_) a group of events spreading outwards from a centre--say, for definiteness, the events constituting a wave of light--and attributed, hypothetically, to a “cause” in that centre; (_b_) more or less similar groups of events at other times, connected with the first group according to the laws of physics, and therefore attributed to the same hypothetical cause at other times. But all that we ought to assume is series of groups of events, connected by discoverable laws. These series we may _define_ as “matter”. Whether there is matter in any other sense, no one can tell. What is true in the old notion of causality is the fact that events at different times are connected by laws (differential equations). When there is a law connecting an event A with an event B, the two have a definite unambiguous time-order. But if the events are such that a ray of light starting from A would arrive at any body which was present at B after B had occurred, and _vice versa_, then there is no definite true order, and no possible causal law connecting A and B. A and B must then be regarded as separate facts of geography. Perhaps the scope and purpose of this and the foregoing chapters may be made clearer by showing their bearing upon certain popular beliefs which may seem self-evident but are really in my opinion either false or likely to lead to falsehood. I shall confine myself to objections which have actually been made to me when trying to explain the philosophical outcome of modern physics.[8] [8] These objections are quoted (with kind permission) from a letter written to me by a well-known engineer, Mr. Percy Griffith, who is also a writer on philosophical subjects.