The possibility of such _a priori_ judgments had hitherto been questioned only by those who sought to deny to them all possible objective validity. Kant, as a rationalist, has no doubt as to their actual existence. In the _Introduction_ to the second edition he bluntly asserts their _de facto_ existence, citing as instances the propositions of mathematics and the fundamental principles of physical science. Their possibility can be accounted for through the assumption of _a priori_ forms and principles.[148] But with equal emphasis he questions the validity of their _metaphysical_ employment. For that is an entirely different matter. We then completely transcend the world of the senses and pass into a sphere where experience can neither guide nor correct us. In this sphere the _a priori_ is illegitimately taken as being at once the source of our professed knowledge and also the sole criterion of its own claims. This is the problem, semi-Critical, semi-dogmatic, which is formulated in the letter of 1772 to Herz.[149] What right have we to regard ideas, which as _a priori_ originate from within, as being valid of things in themselves? In so doing we are assuming a pre-established harmony between our human faculties and the ultimately real; and that is an assumption which by its very nature is incapable of demonstration. The proofs offered by Malebranche and by Leibniz are themselves speculative, and consequently presuppose the conclusion which they profess to establish.[150] As above stated, Kant obtained his answer to this problem by way of the logical enquiry into the nature and conditions of _a priori_ judgment. One of the chief causes, Kant declares, why hitherto metaphysical speculation has passed unchallenged among those who practise it, is the confusion of two very different kinds of judgment, the analytic and the synthetic. Much the greater portion of what reason finds to do consists in the analysis of our concepts of objects. “As this procedure yields real knowledge _a priori_, which progresses in secure and useful fashion, reason is so far misled as surreptitiously to introduce, without itself being aware of so doing, assertions of an entirely different order, in which reason attaches to given concepts others completely foreign to them--and moreover attaches them _a priori_. And yet one does not know how reason comes to do this. This is a question which is never as much as thought of.”[151]