At this point I propose to make a brief digression on the subject of our evidence for causal laws. The laws for which we first get evidence are such as do not hold always, but only as a general rule. As a rule, when you decide to move your arm, it moves: but sometimes it is paralysed and remains motionless. As a rule, when you say how-do-you-do to an old friend, he says the same to you; but he may have grown blind and deaf since you last saw him, and not notice your words or gesture. As a rule, if you put a match to gunpowder, it explodes; but it may have got damp. It is such common but not invariable rules of sequence that we notice first. But science is always seeking to replace them by laws that may have no exceptions. We notice first that heavy bodies fall, then that some bodies do not fall. Then we generalise both sets of facts into the law of gravitation and the laws of resistance of the air. These more general laws do not state that anything will actually happen: they state a tendency, and lead to the conclusion that what actually happens is the resultant of a number of tendencies. We cannot know what the resultant will be unless we know a great deal about the neighbourhood concerned. For example, I might, within the next few seconds be hit on the head by a meteorite; to know whether this is going to happen, I must know what matter is to be found in the neighbourhood of the earth. This illustrates that actual predictions based upon laws which are perfectly valid may always be falsified by some unknown fact of what we may call geography. Moreover, we can never be _sure_ that our scientific laws are quite right; of this the Einsteinian modification of the law of gravitation has afforded a notable instance. Let us now return to the relation between perception and the causal laws of physics. Having realised the abstractness of what physics has to say, we no longer have any difficulty in fitting the visual sensation into the causal series. It used to be thought “mysterious” that purely physical phenomena should end in something mental. That was because people thought they knew a lot about physical phenomena, and were sure they differed in quality from mental phenomena. We now realise that we know nothing of the intrinsic quality of physical phenomena except when they happen to be sensations, and that therefore there is no reason to be surprised that some are sensations, or to suppose that the others are totally unlike sensations. The gap between mind and matter has been filled in, partly by new views of mind, but much more by the realisation that physics tells us nothing as to the intrinsic character of matter.