I call a _cause_, in the narrowest sense of the word, that state of matter, which, while it introduces another state with necessity, yet suffers just as great a change itself as that which it causes; which is expressed in the rule, “action and reaction are equal.” Further, in the case of what is properly speaking a cause, the effect increases directly in proportion to the cause, and therefore also the reaction. So that, if once the mode of operation be known, the degree of the effect may be measured and calculated from the degree of the intensity of the cause; and conversely the degree of the intensity of the cause may be calculated from the degree of the effect. Such causes, properly so called, operate in all the phenomena of mechanics, chemistry, and so forth; in short, in all the changes of unorganised bodies. On the other hand, I call a _stimulus_, such a cause as sustains no reaction proportional to its effect, and the intensity of which does not vary directly in proportion to the intensity of its effect, so that the effect cannot be measured by it. On the contrary, a small increase of the stimulus may cause a very great increase of the effect, or conversely, it may eliminate the previous effect altogether, and so forth. All effects upon organised bodies as such are of this kind. All properly organic and vegetative changes of the animal body must therefore be referred to stimuli, not to mere causes. But the stimulus, like every cause and motive generally, never determines more than the point of time and space at which the manifestation of every force is to take place, and does not determine the inner nature of the force itself which is manifested. This inner nature we know, from our previous investigation, is will, to which therefore we ascribe both the unconscious and the conscious changes of the body. The stimulus holds the mean, forms the transition between the motive, which is causality accompanied throughout by knowledge, and the cause in the narrowest sense. In particular cases it is sometimes nearer a motive, sometimes nearer a cause, but yet it can always be distinguished from both. Thus, for example, the rising of the sap in a plant follows upon stimuli, and cannot be explained from mere causes, according to the laws of hydraulics or capillary attraction; yet it is certainly assisted by these, and altogether approaches very near to a purely causal change. On the other hand, the movements of the _Hedysarum gyrans_ and the _Mimosa pudica_, although still following upon mere stimuli, are yet very like movements which follow upon motives, and seem almost to wish to make the transition. The contraction of the pupils of the eyes as the light is increased is due to stimuli, but it passes into movement which is due to motive; for it takes place, because too strong lights would affect the retina painfully, and to avoid this we contract the pupils. The occasion of an erection is a