"354 W. MCDOUGALL : higher-level system B following immediately upon the ex- citement of any other such system A, may cause the trans- mission of an impulse or flow of energy from A to B, it is clear that B must in some way guide the discharge of the impulse from A, must attract to itself the energy liberated in A, so determining it to take just one path, the path A B, -of all the thousand-fold paths open to it. The generalised ^statement of this conclusion I have elsewhere proposed -to call the law of the attraction of the impulse, 1 and I have pointed out that it is necessarily implied in the wider law of the for- mation of neural habits. The acceptance of any view of the inhibitory process, other than the one here advocated, leaves 'this attraction of the impulse utterly mysterious, while on the other hand the acceptance of the hypothesis of inhibition by 'drainage and its application to the higher-levels of the brain explains at once the attraction of the impulse from the in- hibited system to the inhibiting system and the consequent -establishment of a path of lowered resistance, an "associa- tion-path," between them. When at any moment a system of sensori-motor arcs of the upper level, B, is thrown into excitement, it becomes a broad path of low resistance for the discharge in the forward or motor direction of the free nervous energy accumulated in the afferent side of the brain, and therefore it tends to drain to itself energy from all parts ; there will be a sudden set of currents towards B from all parts at the moment that B is thrown open, as it were, in virtue of its excitement ; and the fullest current will set towards it from that part in which energy is being most abundantly liberated, namely the system A which was pre- dominantly excited at the previous moment, and this fullest -current will flow along any channel between A and B which is most open to it and will in some degree canalise it or leave
- it a still more open path for the future.
Summing up, we may say that when the attention turns from any presentation a to any other b, the presentation a is inhibited and there results an association between a and b such that a tends to reproduce b. On the neural side these two effects, the inhibition and the formation of the associa- tion, are two effects of the one process, the attraction to the neural system B of the free-energy of the system A. The hypothesis of inhibition by drainage seems, then, to explain very well the way in which the energy of the mind is concentrated upon the object of attention, and how the keen- mess of attention depends upon the general awakeness of the 1 Primer of Physiological Psychology, London, 1905, pp. 126, 134.
