Such were the impressions out of which thinking men were obliged to fashion their world-philosophy, while, at the same time, they were members of the possessing classes and had thus an opportunity to employ their leisure for a certain self-study, without, however, being in touch with the source of their impressions, viz., the process of social labor which alone could have enabled them to see through the social origin of their ideas. Men of this class, therefore, were led to the assumption that their ideas emanated from some supernatural and spiritual power or that they were themselves independent supernatural powers. This dualist metaphysical mode of thought has gone through various transformations in the course of time, adapting itself to the evolution of production beginning with ancient slavery, on through the serfdom of the Middle Ages and of mediaeval commodity production, to modern capitalism. These successive changes of form are embodied in Grecian philosophy, in the various phases of the Christian religion, and in the modern systems of philosophy. But we must not regard these systems and religions for what they generally pass, that is to say, we must not think them to be only repeated unsuccessful attempts to formulate absolute truth. They are merely the incarnations of progressive stages of better knowledge acquired by the human mind about itself and about the universe. It was the aim of philosophical thought to find satisfaction in understanding. And as long as understanding could not wholly be gotten by natural means, there remained always a field for the supernatural and incomprehensible. But by the painstaking mental work of the deepest thinkers, the material of science was ceaselessly increased, and the field of the supernatural and incomprehensible was ever more narrowed. And this is especially the case since the progress of capitalist production has promoted the persistent study of nature. For through this study the human mind was enabled to test its powers by simple, quiet, persistent and fruitful labor in the search for successive parts of truth; and thus to rid itself from the overirritation of hopeless quest after absolute truth. The desire to ascertain the value of these new truths gave rise to the problems of the theory of understanding. The attempts to solve these problems form a permanent part of modern systems of philosophy, which represent a graduated evolution of the theory of understanding. But the supernatural element in these systems prevented their perfection.