Metaphysics, as a natural predisposition of reason, is actual, but it is also of itself dialectical and deceitful. In order that metaphysics might, as science, be able to lay claim, not merely to deceitful persuasion, but to insight and conviction, a critique of reason itself must set forth the entire stock of a priori concepts, their division according to the different sources.

Critique stands to the ordinary school metaphysics precisely as chemistry stands to alchemy, or astronomy to the fortune-teller’s astrology. There is an advantage upon which metaphysics alone, among all the possible sciences, can rely with confidence, namely, that it can be completed and brought into a permanent state, since it cannot be further changed and is not susceptible to any augmentation through new discoveries.

There will be metaphysics in the world at every time, and what is more, in every human being, and especially the reflective ones; metaphysics that each, in the absence of a public standard of measure, will carve out for themselves in their own manners. Now what has hitherto been called metaphysics can satisfy no inquiring mind, and yet it is also impossible to give up metaphysics completely; therefore, a critique of pure reason itself must finally be attempted, or, if one exists, it must be examined and put to a general test.

But up to now metaphysics as science has never existed at all. In case the challenge of inquiring how metaphysics as a science is possible is accepted, I must forbid only two things: first, the plaything of probability and conjecture, which suits metaphysics just as poorly as it does geometry; second, decision by means of the divining rod of so-called sound common sense, which does not bend for everyone, but is guided by personal qualities.

As regards the first, there can be nothing more absurd than to want to base one’s judgments in metaphysics, a philosophy from pure reason, on probability and conjecture. Everything that is to be cognized a priori is for that very reason given out as apodictically certain and must therefore also be proven as such. One might just as well want to base a geometry or an arithmetic on conjectures; for as concerns the calculus probabilium of arithmetic, it contains not probable but completely certain judgments about the degree of possibility of certain cases under given homogeneous conditions.

Matters are, if possible, even worse with the appeal to sound common sense, if the discussion concerns concepts and principles, not insofar as they are supposed to be valid with respect to experience, but rather insofar as they are to be taken as valid beyond the conditions of experience. For what is sound common sense? It is the ordinary understanding,2 insofar as it judges correctly. And what now is the ordinary understanding? It is the faculty of cognition and of the use of rules in concreto, as distinguished from the speculative understanding, which is a faculty of the cognition of rules in abstracto. Common sense, or ordinary understanding, will hardly be able to understand the rule: that everything which happens is determined by its cause, and it will never be able to have insight into it in such a general way. But meta- physics is concerned indeed solely with these a priori rules of understanding.

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