Obviously, what man required was a system of mental symbols of some kind or other in the inner world of mind, to represent the system of actual types in the outward world of sense. Each species or type of object in the outward world of nature would require its corresponding symbol in the inward world of mind. Without such mental symbols no articulated advance could be made in that process which we now speak of familiarly as the accumulation of the knowledge of the world. The advance could be made only step by step, and each step would have to be recorded, registered, and fixed by means of its symbol within the rising mental fabric. The discovery or creation of adequate symbols of reason, then, was an obvious necessity for man even to get definitely started at all upon the elaboration or actualization of the world of mind.