ASSOCIATION OF IDEAS. 67 



of photographic drawing are, in their degree, examples of 

 this kind.' l 



All nerve-matter is not only highly impressible, but 

 also retentive of the capacity of reproducing the impres- 

 sion. The great distinction between these actions in 

 inanimate bodies and that of memory is, that the latter 

 also includes recognition of agreement with previous im- 

 pressions, and this is effected by the powers of observation 

 and comparison. We possess a great number of repro- 

 ducible mental impressions, without being aware of it. 

 Of the multitude of impressions we consciously experi- 

 ence, we retain in a permanent form the ideas of only a 

 small proportion. The ideas we permanently retain are 

 either blindly received or intelligently selected ; and we 

 knowingly retain in a latent state in our memory only a 

 very small part of them. As we can only have con- 

 sciously present in our mind a few ideas at a time, nearly 

 all our -mental possessions are latent, and nearly the whole 

 of our ideas are stored up in a hidden and potential state 

 in our brains, ready for use on future occasions. 



We usually associate in thought the ideas of things 

 which are associated in nature ; for instance, redness with 

 copper, hardness with iron, elasticity with steel, india- 

 rubber, and gases ; and we do this in accordance with 

 what has been termed the ' law of contiguity.' We also 

 associate together in thought, in accordance with what 

 has been termed the ' law of similarity,' ideas of things 

 which are similar, but are not necessarily associated in 

 nature. Thus the idea of redness we associate with the 

 ideas of all things we know to be red ; hardness with 

 those of all things we know to be hard, and so on. In 



1 J. W. Draper, M.D., Human Physiology, p. 288. New York, 

 1856. 



F 2 



