In every explanation of pleasure-pain, attention must therefore be paid not only to the claims which are made on the several organs by the objective causes (stimuli or movements), but also to the capacity of the organs to meet these claims. This capacity, on the other hand, is evidently dependent upon the supply of energy afforded by the nutritive processes. In the endeavour to pay due attention to both these factors Lehmann has been led to this conclusion: “Pleasure and pain may in all cases be assumed to be the psychical outcome of the relation between the consumption of energy which at a given moment is demanded from the organs, and the supply of energy which is afforded by nutrition.”[48] In the course of a lengthy and laborious investigation Marshall has arrived at a very similar result: “Pleasure and pain are determined by the relation between the energy given out and the energy received at any given moment by the physical organs which determine the content of that moment.” Pleasure is experienced, according to Marshall’s definition, whenever a surplus of stored energy is discharged in the reaction to the stimulus; pain is experienced whenever a stimulus claims a greater development of energy in the reaction than the organ is capable of affording.[49] In this mode of treatment due attention is paid to those theories according to which the conditions of pleasure are to be sought, not in expenditure of force, but in the receiving of force or in the recovery of balance.[50] But though this point has its own importance, it cannot by any means be put on a level with the dynamic aspect. Pleasure _can never arise_ when the organs are not well-nourished, strong, and capable of function; but it arises only _on the condition_ that they actually do perform a function. As Marshall has rightly remarked, there is no reason to believe that surplus of vigour and receipt of nourishment in themselves could ever be objects of consciousness.[51] As long as we can speak of mental states, these must be accompanied by corresponding activities. The chief merit of Marshall’s thesis is precisely this, that every emotional state, independently of its tone and of its perceptible manifestations, can be interpreted in terms of activity. Pleasure, acute or massive, appears as the result of a stimulus, which, owing to a happy proportion between its intensity and the functional capacity of the organ, has modified the bodily functions in such a way as to produce manifestations of energy. Pain, acute or massive, appears as the result of a stimulus, possibly of the same kind, which, owing to a disproportion arising from its own greater intensity or the smaller functional capacity in the organ, has called forth a functional inhibition—that is to say, that kind of activity which is manifested to us as an arrest of energy.