The present essay attempts to prove that all the principles which we need in order to explain our world and make it intelligible, are within reach of our thought. Thus, the assumption of explanatory principles lying outside our world turns out to be the prejudice of an extinct philosophy which lived on vain dogmatic fancies. This ought to have been Kant's conclusion, too, if he had really enquired into the powers of human thought. Instead, he demonstrated in the most complicated way that the constitution of our cognitive faculties does not permit us to reach the ultimate principles which lie beyond our experience. But we have no reason whatever for positing these principles in any such Beyond. Thus Kant has indeed refuted "dogmatic" philosophy, but he has put nothing in its place. Hence, all German philosophy which succeeded Kant has evolved everywhere in opposition to him. Fichte, Schelling, Hegel simply ignored the limits fixed by Kant for our knowledge and sought the ultimate principles, not beyond, but within, the world accessible to human reason. Even Schopenhauer, though he does declare the conclusions of Kant's Critique of Pure Reason to be eternal and irrefutable truths, cannot avoid seeking knowledge of the ultimate grounds of the world along paths widely divergent from those of his master. But the fatal mistake of all these thinkers was that they sought knowledge of ultimate truths, without having laid the foundation for such an enterprise in a preliminary investigation of the nature of knowledge itself. Hence, the proud intellectual edifices erected by Fichte, Schelling and Hegel have no foundation to rest on. The lack of such foundations reacts most unfavourably upon the arguments of these thinkers. Ignorant of the importance of the world of pure ideas and of its relation to the realm of sense-perception, they built error upon error, one-sidedness upon one-sidedness. No wonder that their over-bold systems proved unable to withstand the storms of an age which recked nothing of philosophy. No wonder that many good things in these systems were pitilessly swept away along with the errors. To remedy the defect which has just been indicated is the purpose of the following investigations. They will not imitate Kant by explaining what our minds can not know: their aim is to show what our minds can know.