to its different possible delimi?tations), and there arises once more a normative and practical logic in the form of an art of thinking, and, especially, of scientific thinking. So far, we are still in the realm of natural thinking. However, the correlation between cognition as mental process, its referent (Bedeutung) and what objectively is, which has just been touched upon in order to contrast the psychology of cognition with pure logic and ontology, is the source of the deepest and most difficult problems. Taken collectively, they are the problem of the possibility of cognition. Cognition in all of its manifestations is a psychic act; it is the cognition of a cognizing subject. The objects cognized stand over and against the cognition. But how can we be certain of the correspondence between cognition and the object cognized? How can knowledge transcend itself and reach its object reliably? The unproblematic manner in which the object of cognition is given to natural thought to be cognized now becomes an enigma. In perception the perceived thing is believed to be directly given. Before my perceiving eyes stands the thing. I see it, and I grasp it. Yet the perceiving is simply a mental act of mine, of the perceiving subject. Likewise, memory and expectation are subjective processes; and so are all thought processes built upon them and through which we come to posit that something really is the case and to determine any truth about what is. How do I, the cognizing subject, know if I can ever really know, that there exist not only my own mental processes, these acts of cognizing, but also that which they apprehend? How can I ever know that there is anything at all which could be set over against cognition as its object? Shall I say: only phenomena are truly given to the cognizing subject, he never does and never can break out of the circle of his own mental processes, so that in truth he could only say: I exist, and all that is not-I is mere phenomenon dissolving into phenomenal connections? Am I then to metaphysics which is bound up with it historically and in subject matter. The task of the theory of knowledge or the critique of theoretical reason is, first of all, a critical one. It must brand the well-nigh inevitable mistakes which ordinary reflection makes about the relation of cognition, its meaning and its object, thereby refuting the concealed as well as the uncon?cealed sceptical theories concerning the essence of cognition by demonstrating their absurdity. Become a solipsist? This is a hard requirement. Shall I, with Hume, reduce