Here, then, we find the essential problem of pure reason. Expressed in a single formula, it runs: How are synthetic _a priori_ judgments possible? To ask this question is to enquire, first, how pure mathematics is possible; secondly, how pure natural science is possible; and thirdly, how metaphysics is possible. That philosophy has hitherto remained in so vacillating a state of ignorance and contradiction is entirely due to the neglect of this problem of _a priori_ synthesis. “Its solution is the question of life and death to metaphysics.” Hume came nearest to realising the problem, but he discovered it in too narrow a form to appreciate its full significance and its revolutionary consequences. “Greater firmness will be required if we are not to be deterred by inward difficulties and outward opposition from endeavouring, through application of a method entirely different from any hitherto employed, to further the growth and fruitfulness of a science indispensable to human reason--a science whose every branch may be cut away but whose root cannot be destroyed.”[154] These statements are decidedly ambiguous, owing to Kant’s failure to distinguish in any uniform and definite manner between immanent and transcendent metaphysics.[155] The term metaphysics is used to cover both. Sometimes it signifies the one, sometimes the other; while in still other passages its meaning is neutral. But if we draw the distinction, Kant’s answer is that a genuine and valid immanent metaphysics is for the first time rendered possible by his _Critique_; its positive content is expounded in the _Analytic_. Transcendent metaphysics, on the other hand, is criticised in the _Dialectic_; it is never possible. The existing speculative sciences transgress the limits of experience and yield only a pretence of knowledge. This determination of the limits of our possible _a priori_ knowledge is the second great achievement of the _Critique_. Thus the _Critique_ serves a twofold purpose. It establishes a new _a priori_ system of metaphysics, and also determines on principles equally _a priori_ the ultimate limits beyond which metaphysics can never advance. The two results, positive and negative, are inseparable and complementary. Neither should be emphasised to the neglect of the other. COMMENT ON THE ARGUMENT OF KANT’S _INTRODUCTION_