I emphasize the inadequacy of all modes of thought and of all understanding in opposition to those who make an idol of science. I emphasize the truth of all perceptions in opposition to those knownothings who claim that truth cannot be understood, but can only be admired and worshipped. Hence it follows for our theory of understanding that intellect and reason and the art of thought are no independent treasure boxes which make any revelations to us. They are theoretical classifications which in practice are operative only in the universal interconnection of things. Understanding, perceiving, judging, distinguishing and concluding, etc., are unable to produce any truths. They can only enlighten and clarify experience by logical classification and distinction. Because man produces works which are preceded by planning, therefore the philosophical mode of research has "assumed that the whole is descended from a thought." But this is an assumption of human origin, which is shown to be without foundation on closer analysis. The plans of our works are copies of natural originals and are "free creations of the mind" only in a limited sense. The artists are well aware of the natural descent of their thoughts and fictions. To regard the world as the outcome of thought is a perverse logic. It is the first condition of rational, proletarian, thought to recognize the intellect and its products as attributes of the world subject. EIGHTEENTH LETTER Just as in political history action and reaction follow one another, just as periods of economic prosperity are alternated by periods of depression, so we find in literature a periodical fluctuation between philosophical and anti-philosophical tendencies. After Hegel had for a time thoroughly aroused the spirits, a time of apathy followed, so that this hero of thought who shortly before had been almost idolized could be attacked and reviled. For about a decade, a philosophical breeze has now once more been blowing. The subject of logic, the theory of understanding, is again the object of universal attention. This movement is stimulated by important discoveries in science, such as the heat equivalent of Robert Mayer, the origin of species by Darwin, etc., and natural science and philosophy may be compared to two miners who are digging a tunnel, so that sharp ears on both sides can hear the blows of the hammers and the clanging of the tools. There is much truth in this picture, but it may also lead to misunderstandings. By the vivisection of frogs and rabbits, by boring into the brain, physiology will not discover the mind. No microscope, no telescope, will reveal the nature of reason and truth or the art of logical discernment. Neither will Lazarre Geiger, Max Mueller, Steinthal, and Noire succeed in philology in solving the "last questions of all knowledge" by the help of any primitive arch-language.