Secondly, we cannot proceed on the assumption that Kant’s maturest teaching comes where, had the _Critique_ been a unitary work, composed upon a definite and previously thought out plan, we should naturally expect to find it, namely, in its concluding portions. The teaching of much of the _Dialectic_, especially in its account of the nature of the phenomenal world and of its relation to the knowing mind, is only semi-Critical. This is also true of Kant’s _Introduction_ to the _Critique_. Introductions are usually written last; and probably Kant’s _Introduction_ was written after the completion of the _Aesthetic_, of the _Dialectic_, and of the _Analytic_ in its earlier forms. But it bears all the signs of having been composed prior to the working out of several of his most characteristic doctrines in the central parts of the _Analytic_. Thus both Kant’s introductory statements of the aims and purposes of the _Critique_, and his application of his results in the solution of metaphysical problems, fail to represent in any adequate fashion the new and revolutionary principles to which he very gradually but successfully worked his way. The key to the _Critique_ is given in the central portions of the _Analytic_, especially in the _Deduction of the Categories_. The other parts of the _Critique_ reveal the Critical doctrines only as gradually emerging from the entangling influence of pre-Critical assumptions. Their teaching has to be radically remodelled before they can be made to harmonise with what, in view both of their intrinsic character and of the corresponding alterations in the second edition, must be regarded as Kant’s maturest utterances. This was a task which Kant never himself attempted. For no sooner had he attained to comparative clearness in regard to his new Critical principles and briefly expounded them in the _Analytic_ of the first edition, than he hastened to apply them in the spheres of morality, aesthetics, and teleology. When the _Critique_ appeared in 1781 he was fifty-seven years of age; and he seems to have feared that if he allowed these purely theoretical problems, which had already occupied his main attention for “at least twelve years,” to detain him longer, he would be debarred from developing and placing on permanent record the new metaphysics of ethics which, as the references in the first _Critique_ show, had already begun to shape itself in his mind. To have expended further energy upon the perfecting of his theoretical philosophy would have endangered its own best fruits. Even the opportunity in 1787 of a second edition of the _Critique_ he used very sparingly, altering or adding only where occasional current criticism--his puzzled contemporaries having still for the most part maintained a discreet silence--had clearly shown that his modes of exposition were incomplete or misleading. II. HISTORICAL KANT’S RELATION TO HUME AND TO LEIBNIZ