I know no investigations more necessary for a full insight into the nature of the faculty which we call _understanding_, and at the same time for the determination of the rules and limits of its use, than those undertaken in the second chapter of the “Transcendental Analytic,” under the title of _Deduction of the Pure Conceptions of the Understanding;_ and they have also cost me by far the greatest labour—labour which, I hope, will not remain uncompensated. The view there taken, which goes somewhat deeply into the subject, has two sides. The one relates to the objects of the pure understanding, and is intended to demonstrate and to render comprehensible the objective validity of its _à priori_ conceptions; and it forms for this reason an essential part of the Critique. The other considers the pure understanding itself, its possibility and its powers of cognition—that is, from a subjective point of view; and, although this exposition is of great importance, it does not belong essentially to the main purpose of the work, because the grand question is what and how much can reason and understanding, apart from experience, cognize, and not, how is the _faculty of thought_ itself possible? As the latter is an inquiry into the cause of a given effect, and has thus in it some semblance of an hypothesis (although, as I shall show on another occasion, this is really not the fact), it would seem that, in the present instance, I had allowed myself to enounce a mere _opinion_, and that the reader must therefore be at liberty to hold a different _opinion_. But I beg to remind him that, if my subjective deduction does not produce in his mind the conviction of its certitude at which I aimed, the objective deduction, with which alone the present work is properly concerned, is in every respect satisfactory.