The behaviour of human beings is distinguished from that of inanimate matter by what are called “mnemic” phenomena, _i.e._ by a certain kind of effect of past occurrences. This kind of effects is exemplified in memory, in learning, in the intelligent use of words, and in every kind of knowledge. But we cannot, on this ground, erect an absolute barrier between mind and matter. In the first place, inanimate matter, to some slight extent, shows analogous behaviour--_e.g._ if you unroll a roll of paper, it will roll itself up again. In the second place, we find that living bodies display mnemic phenomena to exactly the same extent to which minds display them. In the third place, if we are to avoid what I have called “mnemic” causation, which involves action at a distance in time, we must say that mnemic phenomena in mental events are due to the modification of the body by past events. That is to say, the set of events which constitutes one man’s experience is not causally self-sufficient, but is dependent upon causal laws involving events which he cannot experience. On the other hand, our knowledge of the physical world is purely abstract: we know certain logical characteristics of its structure, but nothing of its intrinsic character. There is nothing in physics to prove that the intrinsic character of the physical world differs, in this or that respect, from that of the mental world. Thus from both ends, both by the analysis of physics and by the analysis of psychology, we find that mental and physical events form one causal whole, which is not known to consist of two different sorts. At present, we know the laws of the physical world better than those of the mental world, but that may change. We know the intrinsic character of the mental world to some extent, but we know absolutely nothing of the intrinsic character of the physical world. And in view of the nature of the inferences upon which our knowledge of physics rests, it seems scarcely possible that we should ever know more than abstract laws about matter.