A man’s knowledge may be said to be mature, in other words, when it has reached the most complete state of perfection to which he, as an individual, is capable of bringing it, when an exact correspondence is established between the whole of his abstract ideas and the things he has actually perceived for himself. His will mean that each of his abstract ideas rests, directly or indirectly, upon a basis of observation, which alone endows it with any real value; and also that he is able to place every observation he makes under the right abstract idea which belongs to it.
The second volume consisted of several essays expanding topics covered in the first. Most important are his reflections on death and his theory on sexuality , which saw it as a manifestation of the whole will making sure that it will live on and depriving humans of their reason and sanity in their longing for their loved ones. Less successful is his theory of genetics : he argued that humans inherit their will, and thus their character, from their fathers, but their intellect from their mothers and he provides examples from biographies of great figures to illustrate this theory.  The second volume also contains attacks on contemporary philosophers such as Fichte , Schelling , and Hegel .