Bearing in mind, according to the contents of the last chapter, that the faculty of knowing, like every other organ, has only arisen for the purpose of self-preservation, and that it therefore stands in a precise relation, admitting of countless gradations, to the requirements of each animal species; we shall understand that plants, having so very much fewer requirements than animals, no longer need any knowledge at all. On this account precisely, as I have often said, knowledge is the true characteristic which denotes the limits of animality, because of the movement induced by motives which it conditions. Where animal life ceases, there knowledge proper, with whose essence our own experience has made us familiar, disappears; and henceforth analogy is our only way of making that which mediates between the influence of the outer world and the movements of beings intelligible to us. The will, on the other hand, which we have recognised as being the basis and kernel of every existing thing, remains one and the same at all times and in all places. Now, in the lower degree occupied by plant-life and by the vegetative life of animal organisms, it is the _stimulus_ which takes the place of knowledge as a means of determining the individual manifestations of this omnipresent will and as a mediator between the outer world and the changes of such a being; finally, in inorganic Nature, it is _physical agency in general_; and when, as here, observation takes place from a higher to a lower degree, both stimulus and physical agency present themselves as substitutes for knowledge, therefore as mere analogues to it. Plants cannot properly be said to perceive light and the sun; yet we see them sensitive in various ways to the presence or absence of both. We see them incline and turn towards the light; and though this movement no doubt generally coincides with their growth, just as the moon's rotation on its axis coincides with its movement round the earth, it nevertheless exists, as well as that of the moon, and the direction of that growth is determined and systematically modified by light, just as an action is determined by a motive, and as the direction of the growth of creeping and clinging plants is determined by the shape and position of the supports they may chance to find. Thus because plants on the whole, still have wants, though not such wants as demand the luxury of a sensorium and an intellect, something analogous has to take the place of these, in order to enable the will to lay hold of, if not to seek out, the satisfactions which offer themselves to it. Now, this analogous substitute is susceptibility for stimuli, and I would express the difference between knowledge and this susceptibility as follows: in knowledge, the motive which presents itself as representation and the act of volition which follows from it, _remain distinctly separate one from the other_, this separation moreover being the more distinct, the greater the perfection of the intellect;--whereas, in mere susceptibility for stimuli, the feeling of the stimulus can no longer be distinguished from the volition it occasions, and they coalesce. In inorganic nature finally, even susceptibility for stimuli, the analogy of which to knowledge is unmistakable, ceases, but the diversity of reaction of each body upon divers kinds of action remains; now, when the matter is considered, as we are doing, in the descending scale, this reaction still presents itself, even here, as a substitute for knowledge. If a body reacts differently, it must have been acted upon differently and that action must have roused a different sensation in it, which with all its dullness has nevertheless a distant analogy to knowledge. Thus when water that is shut up finds an outlet of which it eagerly avails itself, rushing vehemently in that direction, it certainly does not recognise that outlet any more than the acid perceives the alkali approaching it which will induce it to abandon its combination with a metal, or than the strip of paper perceives the amber which attracts it after being rubbed; yet we cannot help admitting that what brings about such sudden changes in all these bodies, bears a certain resemblance to that which takes place within us, when an unexpected motive presents itself.