The most perfect and satisfactory knowledge is that of perception, but it is limited absolutely to the particular, the individual. The combination of the many and the different in _one_ idea is only possible through the _conception_, that is, through the omission of the differences; therefore this is a very imperfect manner of presenting things to the mind. Certainly the particular also can be directly comprehended as a universal, if it is raised to the (Platonic) Idea; but in this process, which I have analysed in the third book, the intellect already passes beyond the limits of individuality, and therefore of time; moreover it is only an exception. These inner and essential imperfections of the intellect are further increased by a disturbance which, to a certain extent, is external to it, but yet is unceasing—the influence exerted by the will upon all its operations whenever it is in any way concerned in their result. Every passion, indeed every inclination and aversion, tinges the objects of knowledge with its colour. Of most common occurrence is the falsifying of knowledge which is brought about by wishes and hopes, for they picture to us the scarcely possible as probable and well nigh certain, and make us almost incapable of comprehending what is opposed to it: fear acts in a similar way; and every preconceived opinion, every partiality, and, as has been said, every interest, every emotion and inclination of the will, acts in an analogous manner. To all these imperfections of the intellect we have finally to add this, that it grows old with the brain, that is, like all physiological functions, it loses its energy in later years, whereby all its imperfections are then much increased.