IV NOMENCLATURE OF MENTAL OPERATIONS 105 



pared to the pulling of a long bell-wire. The 

 impulse takes a little time to reach the bell ; the 

 bell rings and then becomes quiescent, until another 

 pull is given. So, in the brain, every sensation is 

 the ring of a cerebral particle, the effect of a 

 momentary impulse sent along a nerve-fibre. 



If there were a complete likeness between the 

 two terms of this very rough and ready comparison, 

 it is obvious that there could be no such thing as 

 memory. A bell records no audible sign of having 

 been rung five minutes ago, and the activity of 

 a sensigenous cerebral particle might similarly 

 leave no trace. Under these circumstances, again, 

 it would seem that the only impressions of relation 

 which could arise would be those of co-existence 

 and of similarity. For succession implies memory 

 of an antecedent state. 1 



But the special peculiarity of the cerebral 

 apparatus is, that any given function which has 

 once been performed is very easily set a-going 

 again, by causes more or less different from those 

 to which it owed its origin. Of the mechanism 

 of this generation of images of impressions or 

 ideas (in Hume's sense), which may be termed 

 Ideation, we know nothing at present, though the 

 fact and its results are familiar enough. 



1 It is not worth while, for the present purpose, to consider 

 whether, as all nervous action occupies a sensible time, the dura- 

 tion of one impression might not overlap that of the impression 

 which follows it, in the case supposed. 



