The products of thinking are concepts and ideas. What a concept is cannot be expressed in words. Words can do no more than draw our attention to the fact that we have concepts. When someone perceives a tree, the perception acts as a stimulus for thought. Thus an ideal element is added to the perceived object, and the perceiver regards the object and its ideal complement as belonging together. When the object disappears from the field of his perception, the ideal counterpart alone remains. This latter is the concept of the object. The wider the range of our experience, the larger becomes the number of our concepts. Moreover, concepts are not by any means found in isolation one from the other. They combine to form an ordered and systematic whole. The concept "organism," e.g., combines with those of "development according to law," "growth," and others. Other concepts based on particular objects fuse completely with one another. All concepts formed from particular lions fuse in the universal concept "lion." In this way, all the separate concepts combine to form a closed, conceptual system within which each has its special place. Ideas do not differ qualitatively from concepts. They are but fuller, more saturated, more comprehensive concepts. I attach special importance to the necessity of bearing in mind, here, that I make thought my starting-point, and not concepts and ideas which are first gained by means of thought. These latter presuppose thought. My remarks regarding the self-dependent, self-sufficient character of thought cannot, therefore, be simply transferred to concepts. (I make special mention of this, because it is here that I differ from Hegel, who regards the concept as something primary and ultimate.) Concepts cannot be derived from perception. This is apparent from the fact that, as man grows up, he slowly and gradually builds up the concepts corresponding to the objects which surround him. Concepts are added to perception.