I. The problems which the critical philosophy undertook definitively to solve may be divided into three classes. The first is the epistemological problem, or the problem offered by human knowledge--its essential nature, its fixed limitations, if such there be, and its ontological validity. It was this problem which Kant brought to the front in such a manner that certain subsequent writers on philosophy have claimed it to be, not only the primary and most important branch of philosophical discipline, but to comprise the sum-total of what human reflection and critical thought can successfully compass. "We call philosophy self-knowledge," says one of these writers. "The theory of knowledge is the true _prima philosophia_," says another. Kant himself regarded it as the most imperative demand of reason to establish a science that shall "determine _a priori_ the possibility, the principles, and the extent of all cognitions." The burden of the epistemological problem has pressed heavily upon the thought of the nineteenth century; the different attitudes toward this problem, and its different alleged solutions, have been most influential factors in determining the philosophical discussions, divisions, schools, and permanent or transitory achievements of the century.
In the epistemological problem as offered by the Kantian philosophy of cognition there is involved the subordinate but highly important question as to the proper method of philosophy. Is the method of criticism, as that method was employed in the three Critiques of Kant, the exclusive, the sole appropriate and productive way of advancing human philosophical thought? I do not think that the experience of the nineteenth century warrants an affirmative answer to this question of method. This experience has certainly, however, resulted in demonstrating the need of a more thorough, consistent, and fundamental use of the critical method than that in which it was employed by Kant. And this improved use of the critical method has induced a more profound study of the psychology of cognition, and of the historical development of philosophy in the branch of epistemology. More especially, however, it has led to the reinstatement of the value-judgments, as means of cognition, in their right relations of harmony with the judgments of fact and of law.