In the Preface to the first edition of Critique of Pure Reason in 1781, Kant wrote that he was “acquainted with no other investigations more important for getting to the bottom of that faculty we call the understanding, and at the same time for the determination of the rules and boundaries of its use, than those I have undertaken in the second chapter of the Transcendental Analytic, under the title Deduction of the Pure Concepts of the Understanding; they are also the investigations that have cost me the most, but I hope not unrewarded, effort” (A xvi). In 1786, in the Preface to the Metaphysical Foundations of Natural Science, Kant lamented that this deduction, “the very part of the Critique that ought to be precisely the most clear,” had instead been found “rather the most obscure,” even circular (MANW, 4:476), and for the second edition of the Critique, a year later, Kant rewrote this chapter completely, hoping to remove its “obscurity” while maintaining that he had “found nothing to alter either in the propositions” of the whole work “or in their grounds of proof” (B xxxvii-xxxviii).