Experimental psychology is a term used with a considerable laxity of content; and so too is that of physiological psychology, or psycho-physics. And the laxity mainly arises because there is an uncertainty as to what is principal and what secondary in the inquiry. Experiment is obviously a help to observation: and so far as the latter is practicable, the former would seem to have a chance of introduction. But in any case, experiment is only a means to an end and only practicable under the guidance of hypothesis and theory. Its main value would be in case the sphere of psychology were completely paralleled with one province of physiology. It was long ago maintained by Spinoza and (in a way by) Leibniz, that there is no mental phenomenon without its bodily equivalent, pendant, or correspondent. The _ordo rerum_ (the molecular system of movements) is, he held, the same as the order of ideas. But it is only at intervals, under special conditions, or when they reach a certain magnitude, that ideas emerge into full consciousness. As consciousness presents them, they are often discontinuous, and abrupt: and they do not always carry with them their own explanation. Hence if we are confined to the larger phenomena of consciousness alone, our science is imperfect: many things seem anomalous; above all, perhaps, will, attention, and the like. We have seen how Herbart (partly following the hints of Leibniz), attempted to get over this difficulty by the hypothesis of idea-forces which generate the forms and matter of consciousness by their mutual impact and resistance. Physiological psychology substitutes for Herbart’s reals and his idea-forces a more materialistic sort of reality; perhaps functions of nerve-cells, or other analogous entities. There, it hopes one day to discover the underlying continuity of event which in the upper range of consciousness is often obscured, and then the process would be, as the phrase goes, explained: we should be able to picture it out without a gap. These large hopes may have a certain fulfilment. They may lead to the withdrawal of some of the fictitious mental processes which are still described in works of psychology. But on the whole they can only have a negative and auxiliary value. The value, that is, of helping to confute feigned connexions and to suggest truer. They will be valid against the mode of thought which, when Psyché fails us for an explanation, turns to body, and interpolates soul between the states of body: the mode which, in an older phraseology, jumps from final causes to physical, and from physical (or efficient) to final. Here, as elsewhere, the physical has its place: and here, more than in many places, the physical has been unfairly treated. But the whole subject requires a discussion of the so-called “relations” of soul and body: a subject on which popular conceptions and so-called science are radically obscure.