It is usual to designate Kant as the founder of the Theory of Knowledge in the modern sense. Against this view it might plausibly be argued that the history of philosophy records prior to Kant numerous investigations which deserve to be regarded as something more than mere beginnings of such a science. Thus Volkelt, in his fundamental work on the Theory of Knowledge,1 remarks that the critical treatment of this discipline took its origin already with Locke. But in the writings of even older philosophers, yes, even in the philosophy of Ancient Greece, discussions are to be found which at the present day are usually undertaken under the heading of Theory of Knowledge. However, Kant has revolutionised all problems under this head from their very depths up, and, following him, numerous thinkers have worked them through so thoroughly that all the older attempts at solutions may be found over again either in Kant himself or else in his successors. Hence, [281]for the purposes of a purely systematic, as distinct from a historical, study of the Theory of Knowledge, there is not much danger of omitting any important phenomenon by taking account only of the period since Kant burst upon the world with his Critique of Pure Reason. All previous epistemological achievements are recapitulated during this period.

The fundamental question of Kant’s Theory of Knowledge is, How are synthetic judgments a priori possible? Let us consider this question for a moment in respect of its freedom from presuppositions. Kant asks the question precisely because he believes that we can attain unconditionally certain knowledge only if we are able to prove the validity of synthetic judgments a priori. He says: “Should this question be answered in a satisfactory way, we shall at the same time learn what part reason plays in the foundation and completion of those sciences which contain a theoretical a priori knowledge of objects;”2 and, further, “Metaphysics stands and falls with the solution of this problem, on which, therefore, the very existence of Metaphysics absolutely depends.”3

Are there any presuppositions in this question, as formulated by Kant? Yes, there are. For the possibility of a system of absolutely [282]certain knowledge is made dependent on its being built up exclusively out of judgments which are synthetic and acquired independently of all experience. “Synthetic” is Kant’s term for judgments in which the concept of the predicate adds to the concept of the subject something which lies wholly outside the subject, “although it stands in some connection with the subject,”4 whereas in “analytic” judgments the predicate affirms only what is already (implicitly) contained in the subject. This is not the place for considering the acute objections which Johannes Rehmke5 brings forward against this classification of judgments. For our present purpose, it is enough to understand that we can attain to genuine knowledge only through judgments which add to one concept another the content of which was not, for us at least, contained in that of the former. If we choose to call this class of judgments, with Kant, “synthetic,” we may agree that knowledge in judgment form is obtainable only where the connection of predicate and subject is of this synthetic sort. But, the case is very different with the second half of Kant’s question, which demands that these judgments are to be formed a priori, i.e., independently of all experience. For one thing, it is altogether possible6 that such judgments do not occur at [283]all. At the start of the Theory of Knowledge we must hold entirely open the question, whether we arrive at any judgments otherwise than by experience, or only by experience. Indeed, to unprejudiced reflection the alleged independence of experience seems from the first to be impossible. For, let the object of our knowledge be what it may—it must, surely, always present itself to us at some time in an immediate and unique way; in short, it must become for us an experience. Mathematical judgments, too, are known by us in no other way than by our experiencing them in particular concrete cases. Even if, with Otto Liebmann,7 for example, we treat them as founded upon a certain organisation of our consciousness, this empirical character is none the less manifest. We shall then say that this or that proposition is necessarily valid, because the denial of its truth would imply the denial of our consciousness, but the content of a proposition can enter our knowledge only by its becoming an experience for us in exactly the same way in which a process in the outer world of nature does so. Let the content of such a proposition include factors which guarantee its absolute validity, or let its validity be based on other grounds—in either case, I can possess myself of it only in one way and in no other: it must be presented to me in experience. This is the first objection to Kant’s view. [284]

The other objection lies in this, that we have no right, at the outset of our epistemological investigations, to affirm that no absolutely certain knowledge can have its source in experience. Without doubt, it is easily conceivable that experience itself might contain a criterion guaranteeing the certainty of all knowledge which has an empirical source.

Thus, Kant’s formulation of the problem implies two presuppositions. The first is that we need, over and above experience, another source of cognitions. The second is that all knowledge from experience has only conditional validity. Kant entirely fails to realise that these two propositions are open to doubt, that they stand in need of critical examination. He takes them over as unquestioned assumptions from the dogmatic philosophy of his predecessors and makes them the basis of his own critical inquiries. The dogmatic thinkers assume the validity of these two propositions and simply apply them in order to get from each the kind of knowledge which it guarantees. Kant assumed their validity and only asks, What are the conditions of their validity? But, what if they are not valid at all? In that case, the edifice of Kantian doctrine lacks all foundation whatever.

The whole argumentation of the five sections which precede Kant’s formulation of the problem, amounts to an attempt to prove that the propositions of Mathematics are [285]synthetic.8 But, precisely the two presuppositions which we have pointed out are retained as mere assumptions in his discussions. In the Introduction to the Second Edition of the Critique of Pure Reason we read, “experience can tell us............