What, then, becomes of the scientific conception of energy? Is not energy that which produces observable change? Is it not active in the sense required? And can we say that this conception is useless for scientific interpretation? I suppose most of us, in our student days, have passed through the phase of regarding energy as an active demon which plays a notorious part in the physical drama. Spencer loved it dearly. But some of us, under what we consider wiser guidance, hold that what we should understand by kinetic energy is nothing of this sort. It is a constant ratio of variables, conveniently expressed as 1/2_mv_^{2}. That, however, it may be said, is absurd. Energy is not merely a ratio or a formula; it is something much more real; perhaps the most real of all the realities the being of which has been disclosed by physical science. Granted in a sense, and a very true sense. But what is this reality? It is the reality of the changes themselves in those fields of relatedness to which the formula has reference. There is nothing, I conceive, in the modern treatment of energy that affords any scientific justification of the Spencerian view[43] that energy is an agent through the activity of which the constant ratio of variables is maintained in the physical world. I feel sure that it will still be said that change must inevitably imply that which produces change, and that, even if energy be only a ratio of variables within a changing field, there is still the implication of Force as the real Cause of which the change itself, however formulated, is the effect. No doubt this is one of the meanings which the ambiguous words force and cause may carry. It is to remove this ambiguity that I have suggested that the word Source should be substituted for cause in this sense. And what about force? In one of its meanings it now generally stands for a measure of change. For those who accept Source as a scientific concept it may well stand for the measure or degree of its activity gauged by the phenomenal effect; for those who do not accept it, the measure or degree of the change itself[44]--to be dealt with in mechanics in terms of mass and acceleration. This leaves outstanding, however, the use of the word force in the phrase--the forces of nature--gravitative force, cohesive force, electromotive force, and so on. It was, I take it, with this usage in view that Spencer spoke of vital, mental, and social forces. Now the reference in each of these cases is to some specific mode of relatedness among the things concerned. We need to name it in some way; and this is the way that is, rather unfortunately, sanctioned by custom and long usage. When we say that a thing is in a field of electromotive force we mean (do we not?) that the relatedness is of that particular kind named electromotive, and not of another kind. When Spencer spoke of social forces he had in view changes which take place within a field of social relationships.