42(1 Dr. G. J. Stoney on the 



when be is tempted either to speak or think of u undifferen- 

 tiated protoplasm." 



4. A still more striking instance is presented when we 

 consider the operations of the human mind. Here I will 

 make the usual assumption, that every perception or other 

 thought in the mind is accompanied by a physical event 

 occurring in the brain, which is connected with it in such a 

 way that neither presents itself without the other. Of this 

 event we know that it is of a kind that arises only in living 

 brains and in them only while the man is either awake or 

 dreaming. We also know that it is of a kind that lasts for a 

 considerable time when it does occur, viz. throughout the 

 duration of the perception or other thought in the mind. 



This last consideration is very significant. The event in 

 the brain with which human perception or any other human 

 thought is associated must be one which can last while the 

 thought lasts, i.e. for a time immensely long when compared 

 with the original molecular events that are going on. The 

 event may, for example, be such an (went as a strain conse- 

 quent on a stress, whether dynamical or electromagnetic, 

 acting on some part of the brain ; or it may be of the nature 

 of a forced vibration or current. These are events which 

 would continue in existence so long as the stress is applied, 

 and will cease when the stress is removed : they fulfil the 

 requisite time conditions. Another event which would fulfil 

 the time conditions is an undulation — dynamical, electro- 

 magnetic, or of any other kind. The waves that make up an 

 undulation may continue in it but a short time, some passing 

 oft' while others come on, and the motions or stresses of which 

 each wave consists may be such as succeed each other with 

 extreme rapidity, while all the time the undulation viewed as 

 a whole continues as much unchanged as a human thought 

 does while it lasts. Hence an event of this kind may, so far 

 as its relation to time is concerned, be that event in the brain 

 which is intimately associated with human thought. Possibly 

 the event we are in search of may be found among the pro- 

 cesses of metabolism whereby nutrient matter brought by the 

 blood becomes part of the brain ; or more probably among 

 those processes in which matter that had formed part of the 

 brain separates and is swept away either by the blood or 

 lymphatic vessels. Events of this kind, including every 

 interference with or modification of those here specified, and 

 the many other events which like them may be described as 

 stream effects, are marked by the peculiarity that a vast 

 number of molecules are concerned in them in such a way 

 that different molecules successively take up the running. 



