insufficiency of rational knowledge to fathom the nature of the world Kant started from knowledge as a _fact_, which our consciousness affords us, thus in this sense he proceeded _a posteriori_. But in this chapter, and also in my work, “_Ueber den Willen in der Natur_,” I have sought to show what knowledge is in its _nature and origin_, something secondary, designed for individual ends; whence it follows that it _must be_ insufficient to fathom the nature of the world. Thus so far I have reached the same goal _a priori_. But one never knows anything wholly and completely until one has gone right round it for that purpose, and has got back to it from the opposite side from which one started. Therefore also, in the case of the important fundamental knowledge here considered, one must not merely go from the intellect to the knowledge of the world, as Kant has done, but also from the world, taken as given, to the intellect, as I have undertaken here. Then this physiological consideration, in the wider sense, becomes the supplement of that ideological, as the French say, or, more accurately, transcendental consideration. In the above, in order not to break the thread of the exposition, I have postponed the explanation of one point which I touched upon. It was this, that in proportion as, in the ascending series of animals, the intellect appears ever more developed and complete, _knowledge_ always separates itself more distinctly from _will_, and thereby becomes purer. What is essential upon this point will be found in my work, “_Ueber den Willen in der Natur_,” under the heading, “_Pflanzenphysiologie_” (p. 68‐72 of the second, and 74‐77 of the third edition), to which I refer, in order to avoid repetition, and merely add here a few remarks. Since the plant possesses neither irritability nor sensibility, but the will objectifies itself in it only as plastic or reproductive power, it has neither muscle nor nerve. In the lowest grades of the animal kingdom, in zoophites, especially in polyps, we cannot as yet distinctly recognise the separation of these two constituent parts, but still we assume their existence, though in a state of fusion; because we perceive movements which follow, not, as in the case of plants, upon mere stimuli, but upon motives, _i.e._, in consequence of a certain apprehension. Now in proportion as, in the ascending series of animals, the nervous and muscular systems _separate_ ever more distinctly from each other, till in the vertebrate animals, and most completely in man, the former divides into an organic and a cerebral nervous system, and of these the latter again develops into the excessively complicated apparatus of the cerebrum and cerebellum, spinal marrow, cerebral and spinal nerves, sensory and motor nerve fascicles, of which only the cerebrum, together with the sensory nerves depending upon it, and the posterior spinal nerve fascicles are intended for the _apprehension of the motive_ from the external world, while all