From the epistemological point of view, error can occur only within the sphere of cognitive activity. An illusion of the senses is no error. The fact that the rising moon appears to us bigger than the moon overhead is not an error, but a phenomenon fully explained by the laws of nature. An error would result only, if thought, in ordering the data of perception, were to put a false interpretation on the "bigger" or "smaller" size of the moon. But such an interpretation would lie within the sphere of cognitive activity. If knowledge is really to be understood in its essential nature, we must, without doubt, begin our study of it at the point where it originates, where it starts. Moreover, it is clear that whatever precedes its starting-point has no legitimate place in any explanatory Theory of Knowledge, but must simply be taken for granted. It is the task of science, in its several branches, to study the essential nature of all that we are here taking for granted. Our aim, here, is not to acquire specific knowledge of this or that, but to investigate knowledge as such. We must first understand the act of cognition, before we can judge what significance to attach to the affirmations about the content of the world which come to be made in the process of getting to know that content. For this reason, we abstain from every attempt to determine what is immediately-given, so long as we are ignorant of the relation of our determinations to what is determined by them. Not even the concept of the "immediately-given" affirms any positive determination of what precedes cognition. Its only purpose is to point towards the Given, to direct our attention upon it. Here, at the starting-point of the Theory of Knowledge, the term merely expresses, in conceptual form, the initial relation of the cognitive activity to the world-content. The choice of this term allows even for the case that the whole world-content should turn out to be nothing but a figment of our own "Ego," i.e., that the most extreme subjectivism should be right. For, of course, subjectivism does not express a fact which is given. It can, at best, be only the result of theoretical considerations. Its truth, in other words, needs to be established by the Theory of Knowledge. It cannot serve as the presupposition of that theory. This immediately-given world-content includes everything which can appear within the horizon of our experience, in the widest sense of this term, viz., sensations, percepts, intuitions, feelings, volitions, dreams, fancies, representations, concepts, ideas. Illusions, too, and hallucinations stand at this level exactly on a par with other elements of the world-content. Only theoretical considerations can teach us in what relations illusions, etc., stand to other percepts.