FORMS OF CONSCIOUSNESS. 253 



conscious life. This very speech of others is a type of activity which 

 we interpret by reference to the modifications of consciousness which 

 go with our own similar speech activities. When, for instance, I say 

 ' yes/ I hear the sound of my own voice and at the same time ex- 

 perience a modification of consciousness which I describe as the state 

 of assent. When my friend says ' yes ' I hear the same sounds which 

 a moment ago proceeded from my own body, and I assume that my 

 friend experiences the same conscious state that I describe as assent. 

 In other words, we introject into other men, as it were, conscious 

 states similar to our own conscious states, when they and we our- 

 selves act in the same way, or are subjected to the same stimulations 

 from the environment. 



Even when we come to an agreement that consciousness exists in 

 each of us we depend upon this interpretation — this argument by 

 analogy — for our simplest knowledge of the mental states of other 

 men. You and I agree to call the conscious states accompanying 

 stimulations of the eye, light sensations; but in the fact that stimuli 

 of the same nature reach my eye and your eye I have no evidence that 

 what you call light sensations are what I call light sensations, apart 

 from the fact that I judge by analogy that, as you are very like me, 

 you are to be credited when you say that you have a consciousness 

 very like mine; and that as your eye is very like mine, its stimulation 

 by light must correspond with modifications of your consciousness 

 very similar to the modifications in my consciousness that correspond 

 with the stimulation of my eye under the same light conditions. 



That this argument by analogy is the basis of our assumption of 

 the existence of consciousness in other men becomes indeed very 

 clear in the fact that we do not hesitate for a moment to pass beyond 

 humankind and ascribe consciousness to the higher animals other 

 than man, although they are entirely incapable of describing their 

 mental states to us. 



I have, of course, no fault to find with this manner of our thought ; 

 I wish, however, in the very beginning to emphasize this fact, for in 

 what follows I shall attempt to show that in connection with certain 

 generally accepted modern views we are led to follow out this argu- 

 ment by analogy much farther than it is commonly carried, and to 

 results which are of very great interest. 



I. Of Consciousnesses Simpler than Human Consciousness. 



As we have noted, the existence of conscious states in connection 

 with animal activities is naturally inferred by each of us. It is also 

 very generally agreed that the mental life of even the highest of 

 animals is simpler than our own. These conclusions were reached 

 long before men had gained any knowledge of the nature of the human 



