But that passage is equally inadequate as a complete expression of Kant’s Critical philosophy. The truth lies midway between it and the teaching of the _Preface_ to the first edition. Pure reason is as defective an instrument of knowledge as is factual experience. Though the primary aim of metaphysics is to determine our relation to the absolutely real, and though that can only be done by first determining the nature and possible scope of _a priori_ principles, such principles are found on investigation to possess only empirical validity. The central question of the _Critique_ thus becomes the problem of the validity of their empirical employment. The interrelation of these two problems, that of the _a priori_ and that of experience, and Kant’s attitude towards them, cannot be considered till later. The defects of the _Preface_ to the first edition are in part corrected by the extremely valuable _Preface_ substituted in the second edition. But some further points in this first _Preface_ must be considered. =Prescribed by the very nature of reason itself.=[89]--Metaphysics exists as a “natural disposition,” and its questions are not therefore merely artificial. “As natural disposition (_Naturanlage_) ... metaphysics is real. For human reason, without being moved merely by the idle desire for extent and variety of knowledge, proceeds impetuously, driven on by an inward need, to questions such as cannot be answered by any empirical employment of reason, or by principles thence derived. Thus in all men, as soon as their reason has become ripe for speculation, there has always existed and will always continue to exist some kind of metaphysics.”[90] Hence results what Kant entitles _transcendental illusion_.