The concept "tree" is conditioned for our knowledge by the percept "tree." There is only one determinate concept which I can select from the general system of concepts and apply to a given percept. The connection of concept and percept is mediately and objectively determined by thought in conformity with the percept. The connection between a percept and its concept is recognised after the act of perception, but the relevance of the one to the other is determined by the character of each. Very different is the result when we consider knowledge, and, more particularly, the relation of man to the world which occurs in knowledge. In the preceding chapters the attempt has been made to show that an unprejudiced examination of this relation is able to throw light on its nature. A correct understanding of this examination leads to the conclusion that thinking may be intuitively apprehended in its unique, self-contained nature. Those who find it necessary, for the explanation of thinking as such, to invoke something else, e.g., physical brain-processes, or unconscious spiritual processes lying behind the conscious thinking which they observe, fail to grasp the facts which an unprejudiced examination yields. When we observe our thinking, we live during the observation immediately within the essence of a spiritual, self-sustaining activity. Indeed, we may even affirm that if we want to grasp the essential nature of Spirit in the form in which it immediately presents itself to man, we need but look at our own self-sustaining thinking. For the study of thinking two things coincide which elsewhere must always appear apart, viz., concept and percept. If we fail to see this, we shall be unable to regard the concepts which we have elaborated in response to percepts as anything but shadowy copies of these percepts, and we shall take the percepts as presenting to us reality as it really is. We shall, further, build up for ourselves a metaphysical world after the pattern of the world of percepts. We shall, each according to his habitual ideas, call this world a world of atoms, or of will, or of unconscious spirit, and so on. And we shall fail to notice that all the time we have been doing nothing but erecting hypothetically a metaphysical world modeled on the world we perceive. But if we clearly apprehend what thinking consists in, we shall recognise that percepts present to us only a portion of reality, and that the complementary portion which alone imparts to reality its full character as real, is experienced by us in the organisation of percepts by thought. We shall regard all thought, not as a shadowy copy of reality, but as a self-sustaining spiritual essence. We shall be able to say of it, that it is revealed to us in consciousness through intuition. Intuition is the purely spiritual conscious experience of a purely spiritual content. It is only through intuition that we can grasp the essence of thinking.