carried over into nature, travelling there side by side with the idea of necessity, has got corrupted before it returns from the journey. It returns impregnated with the idea of necessity: and in the light of the rôle which we have made it play in the external world, we regard force as determining with strict necessity the effects which flow from it. Here again the mistake made by consciousness arises from the fact that it looks at the self, not directly, but by a kind of refraction through the forms which it has lent to external perception, and which the latter does not give back without having left its mark on them. A compromise, as it were, has been brought about between the idea of force and that of necessary determination. The wholly mechanical determination of two external phenomena by one another now assumes in our eyes the same form as the dynamic relation of our exertion of force to the act which springs from it: but, in return, this latter relation takes the form of a mathematical derivation, the human action being supposed to issue mechanically, and therefore necessarily, from the force which produces it. There is no doubt that this mingling of two different and almost opposite ideas offers advantages to common sense, since it enables us to picture in the same way, and denote by one and the same word, both the relation which exists between two moments of our life and that which binds together the successive moments of the external world. We have seen that, though our deepest conscious states exclude numerical multiplicity, yet we break them up into parts external to one another; that though the elements of concrete duration permeate one another, duration expressing itself in extensity exhibits moments as distinct as the bodies scattered in space. Is it surprising, then, that between the moments of our life, when it has been, so to speak, objectified, we set up a relation analogous to the objective relation of causality, and that an exchange, which again may be compared to the phenomenon of endosmosis, takes place between the dynamic idea of free effort and the mathematical concept of necessary determination? [Sidenote: Though united in popular thought, the ideas of free effort and necessary determination are kept apart by physical science.] But the sundering of these two ideas is an accomplished fact in the natural sciences. The physicist may speak of _forces,_ and even picture their mode of action by analogy with an inner effort, but he will never introduce this hypothesis into a scientific explanation. Even those who, with Faraday, replace the extended atoms by dynamic points, will treat the centres of force and the lines of force mathematically, without troubling about force itself considered as an activity or an effort. It thus comes to be understood that the relation of external causality is purely mathematical, and has no resemblance to the relation between psychical force and the act which springs from it.