ASSOCIATION OF IDEAS. 119 



Basis of ]\Iiricl) that ideas have a strong tendency to cohere 

 together in groups, so as to constitute one compound idea 

 out of many simpler or more elementary ideas ; and also 

 that they show no less strong a tendency to coliere together 

 in concatenated series, such that the arousing of the first 

 member determines the successive arousing of the other 

 members. On its physiological side, as we saw, this is pre- 

 cisely analogous on the one hand to the co-ordination of 

 muscular movements in space (i.e., the grouping of such 

 movements to form a simultaneous act, such as striking), and 

 on tlie other hand to the co-ordination of muscular movements 

 in time (i.e., the grouping of such movements to perform a 

 serial act, such as vomiting). Now it is found by observa- 

 tion that this cohesion of ideas is determined either by con- 

 tiguity or by similarity. This fact is too well and generally 

 known to call for more than a bare statement. 



Association by contiguity is more primitive than associa- 

 tion by the similarity, for in order that there should be asso- 

 ciation by similarity, the similarity must be iMrceived ; and 

 this implies a higher level of mental evolution than is 

 required to establish an association by contiguity — which, as 

 we have seen, may be established even in non-mental nervous 

 processes, while there is nothing truly analogous to associa- 

 tion by similarity observable in such processes.* 



But it will be observed that even association of ideas by 

 contiguity of the simplest possible kind, implies a higher 

 development of the powers of memory than any of the three 

 stages of memory which I have already indicated. For now 

 there is not merely the memory of a past sensation (which is 

 dormant till aroused by another like or unlike sensation) ; 

 but there is the memory of at least two things, and also the 

 memory of a previous relation of sequence between them. 



* The nearest approach to such an analogy is perhaps to be found in the 

 curious fact, which I find to hold true in most persons, that if a pencil is 

 taken in each hand, and while the habitual signature is being written with 

 the right hand, moving from left to right, the movements are imitated by the 

 left hand moving in the opposite direction, the signature will be found to 

 bave been written backwards by the left hand, and even tbe hand-writing can 

 be recognized on holding the paper before a mirror. As the left hand may 

 never have performed this feat before, and cannot perform it unless the right 

 hand is working simultaneously, the case looks like one of association by 

 similarity. But I think it is really due to association by contiguity ; and 

 the same applies to the extreme difficulty of moving the two hands simul- 

 taneously as if carding wool in opposite directions. 



