Several centuries ago it occurred to the man of science that the material world should be regarded as a system in which there is constant transformation, but in which nothing is created. This way of looking at things expressed itself formerly in the statement that, through all the changes that take place in the world, the quantity of matter and motion remains the same. To-day the same idea is better expressed in the doctrine of the eternity of mass and the conservation of energy. In plain language, this doctrine teaches that every change in every part of the physical world, every motion in matter, must be preceded by physical conditions which may be regarded as the equivalent of the change in question. But this makes the physical world a closed system, a something complete in itself. Where is there room in such a system for minds? It does indeed seem hard to find in such a system a place for minds, if one conceives of minds as does the interactionist. We have seen (section 36) that the interactionist makes the mind act upon matter very much as one particle of matter is supposed to act upon another. Between the physical and the mental he assumes that there are _causal_ relations; _i.e._ physical changes must be referred to mental causes sometimes, and mental changes to physical. This means that he finds a place for mental facts by inserting them as links in the one chain of causes and effects with physical facts. If he is not allowed to break the chain and insert them, he does not know what to do with them. The parallelist has not the same difficulty to face. He who holds that mental phenomena must not be built into the one series of causes and effects with physical phenomena may freely admit that physical phenomena form a closed series, an orderly system of their own, and he may yet find a place in the world for minds. He refuses to regard them as a part of the world-mechanism, but he _relates_ them to physical things, conceiving them as _parallel to_ the physical in the sense described (sections 37-39). He insists that, even if we hold that there are gaps in the physical order of causes and effects, we cannot conceive these gaps to be filled by mental phenomena, simply because they are mental phenomena. They belong to an order of their own. Hence, the assumption that the physical series is unbroken does not seem to him to crowd mental phenomena out of their place in the world at all. They must, in any case, occupy the place that is appropriate to them (section 38).