In this chapter, the author considers what we can know of other minds than ours, and how we may gain this knowledge. It follows from what has just been said, that since biological evolution has given rise to individuals of divergent types of organic structure. There may be—nay, there must be—in these divergent biological individuals divergent types of mind, using the word "mind" in the widest and most comprehensive sense as embracing all modes of psychical activity. The author illustrates the fundamental difficulty of comparative psychology by means of an analogy. Suppose that a chronometer were gifted with intelligence and reason, and were to enter upon the study of other timepieces, all access to their works being inexorably denied it. It would be able to observe the motions of the hands over the dial-plate, and perhaps gain some information by attentively listening to internal sounds.