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 experience 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 ourselves 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 argument 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
