The exposition given in our third book of the knowledge of the (Platonic) Ideas, as the highest attainable by man, and at the same time entirely _perceptive or intuitive_ knowledge, is a proof that the source of true wisdom does not lie in abstract rational knowledge, but in the clear and profound apprehension of the world in perception. Therefore wise men may live in any age, and those of the past remain wise men for all succeeding generations. Learning, on the contrary, is relative; the learned men of the past are for the most part children as compared with us, and require indulgence. But to him who studies in order to gain _insight_ books and studies are only steps of the ladder by which he climbs to the summit of knowledge. As soon as a round of the ladder has raised him a step, he leaves it behind him. The many, on the other hand, who study in order to fill their memory do not use the rounds of the ladder to mount by, but take them off, and load themselves with them to carry them away, rejoicing at the increasing weight of the burden. They remain always below, because they bear what ought to have borne them. Upon the truth set forth here, that the kernel of all knowledge is the _perceptive or intuitive_ apprehension, depends the true and profound remark of Helvetius, that the really characteristic and original views of which a gifted individual is capable, and the working up, development, and manifold application of which is the material of all his works, even if written much later, can arise in him only up to the thirty-fifth or at the latest the fortieth year of his life, and are really the result of combinations he has made in his early youth. For they are not mere connections of abstract conceptions, but his own intuitive comprehension of the objective world and the nature of things. Now, that this intuitive apprehension must have completed its work by the age mentioned above depends partly on the fact that by that time the ectypes of all (Platonic) Ideas must have presented themselves to the man, and therefore cannot appear later with the strength of the first impression; partly on this, that the highest energy of brain activity is demanded for this quintessence of all knowledge, for this proof before the letter of the apprehension, and this highest energy of the brain is dependent on the freshness and flexibility of its fibres and the rapidity with which the arterial blood flows to the brain. But this again is at its strongest only as long as the arterial system has a decided predominance over the venous system, which begins to decline after the thirtieth year, until at last, after the forty-second year, the venous system obtains the upper hand, as Cabanis has admirably and instructively explained. Therefore the years between twenty and thirty and the first few years after thirty are for the intellect what May is for the trees; only then do the blossoms appear of which all the later fruits are the development.