1. Every empirical science has, as its primary and characteristic subject of treatment, certain particular facts of experience whose nature and reciprocal relations it seeks to investigate. In solving these problems it is found to be necessary, if we try not to give up entirely the grouping of the facts under leading heads, to have general supplementary concepts that are not contained in experience itself, but are gained by a process of logical treatment of this experience. The most general supplementary concept of this kind that has found its place in all the empirical sciences, is the concept of causality. It comes from the necessity of thought. that all our experiences shall be arranged according to reason and consequent, and that we shall remove, by means of second" supplementary concepts and if need be by means of concepts of a hypothetical character, all contradictions that stand in the way of the establishment of a consistent interconnection of this kind. In this sense we may regard all the supplementary concepts that serve for the interpretation of any sphere of experience, as applications of the general principle of causation. They are justified in so far as they are required, or at least rendered probable, by this principle; they are unjustifiable so soon as they prove to be arbitrary [p. 311] fictions resulting from foreign motives, and contributing nothing to the interpretation of experience.
2. In this sense the concept matter is a fundamental supplementary concept of natural science. In its most general significance it designates the permanent substratum assumed as existing in universal space, to whose activities we must attribute all natural phenomena. In this most general sense the concept matter is indispensable to every explanation of natural science. The attempt in recent times to raise energy to the position of a governing principle, does not succeed in doing away with the concept matter, but merely gives it a different content. This content, however, is given to the concept by means of a second supplementary concept, which relates to the causal activity of matter. The concept of matter that has been accepted in natural science up to the present time, is based upon the mechanical physics of Galileo, and uses as its secondary supplementary concept the concept of force which is defined as the product of the mass and the momentary acceleration. A physics of energy would have to use everywhere instead of this the concept energy, which in the special form of mechanical energy is defined as half the product of the mass multiplied by the square of the velocity. Energy, however, must, just as well as force, have a position in objective space, and under certain particular conditions the points from which energy proceeds may, just as well as the .points from which force proceeds, change their place in space, so that the concept of matter as a substratum contained in space, is retained in both cases. The only difference, and it is indeed an important one, is that when we use the concept force, we presuppose the reducibility of all, natural phenomena to forms of mechanical motion, while when we use the concept of energy, we attribute to matter not only the property of motion without a change in the form of [p. 312]energy, but also the property of the transformability of qualitatively different forms of energy into one another without a change in the quantity of the energy.
3. The concept of mind is a supplementary concept of psychology, in the same way that the concept matter is supplementary concept of natural science. It too is indispensable in so far as we need a concept which shall express in a comprehensive way the totality of psychical experiences in an individual consciousness. The particular content of the concept, however, is in this case also entirely dependent on the secondary concepts that give a more detailed definition of psychical causality. In the definition of this content psychology shared at first the fortune of the natural sciences.
Both the concept of mind and that of matter arose primarily not so much from the need of explaining experience as from the effort to reach a systematic doctrine of the general interconnection of all things. But while the natural sciences have long since outgrown this mythological stage of speculative definition, and make use of some of the single ideas that originated] at that time, only for the purpose of gaining definite starting-points for a strict methodical definition of their concepts, psychology has continued under the control of the mythological, metaphysical concept of mind down to most modern times, and still remains, in part at least, under its control.
This concept is not used as a general supplementary concept that serves primarily to gather together the psychical facts and only secondarily to give a causal interpretation of them but it is employed as a means to satisfy so far as possible the need of a general universal system, including both nature and the individual existence.
4. The concept of a mind-substance in its various forms is rooted in this mythological and metaphysical need. In its development there have not been wanting efforts to meet [p. 313] from this position, so far as possible, the demand for a psychological causal explanation, still, such efforts have in all cases been afterthoughts; and it is perfectly obvious that psychological experience alone, independent of all foreign metaphysical motives, would never have led to a concept of mind-substance. This concept has beyond a doubt exercised a harmful influence on the treatment of experience.