THE FACTS OF SCIENCE 45 



case to answer their demands, but this may require him 

 to listen to the special communications of these subscribers, 

 to examine his lists, his post-office directory, or any other 

 source of information stored in his office. Finally, he 

 shunts their wires so as to bring them in circuit with those 

 of Y and Z, which seem to best suit the nature of the 

 demands. This corresponds to an exertion following 

 consciously on the receipt of a sense-impression. In all 

 cases the activity of the exchange arises from the receipt 

 of a message from one of a possibly great but still finite 

 number of senders. A, B, C, D, etc. ; the originality of 

 the clerk is confined to immediately following their behests 

 or to satisfying their demands to the best of his ability by 

 the information stored in his office. The analogy, of 

 course, must not be pressed too far — in particular, senders 

 and receivers must be considered distinct, for sensory and 

 motor nerves do not appear to interchange functions. 

 But the conception of the brain as a central exchange 

 certainly casts considerable light not only on the action 

 of sensory and motor nerves, but also on thought and 

 consciousness. Without sense -impressions there would 

 be nothing to store ; without the faculty of receiving 

 permanent impress, without memory, there would be no 

 possibility of thought ; and without this thought, this 

 period of hesitation between sense-impression and exertion, 

 there would be no consciousness. When an exertion 

 follows immediately on a sense-impression we speak of 

 the exertion as involuntary, our action as subject to the 

 mechanical control of the " external object " to which we 

 attribute the sense-impression. On the other hand, when 

 the exertion is conditioned by stored sense-impresses we 

 term our action voluntary. We speak of it as determined 

 from " within ourselves," and assert the " freedom of our 

 will." In the former case the exertion is conditioned 

 solely by the immediate sense-impression ; in the latter it 

 is conditioned by a complex of impressions partly im- 

 mediate and partly stored. The past training, the past 

 history and experience which mould character and de- 

 termine the will, are really based on sense-impressions 



