ON IDEAS. 35 



system. This selective power is like that of a bone, 

 muscle, or nerve, each of which will only take to itself 

 from the blood its own proper ingredients. Brewster ob- 

 served that ' In the complex formation of apophyllite and 

 analcime, laws operate more like those in living structures 

 than in crystalline formations.' l The property possessed 

 by substances in general of enabling the application of 

 one force to produce a number of effects simultaneously, 

 may also be regarded as additional evidence of the great 

 complexity of material substances. 



We may conclude from these and many other similar 

 facts, that we are surrounded on the one hand by pheno- 

 mena of almost infinite magnitude, and on the other by 

 an endless number of others of almost infinite minuteness, 

 and complexity. 



CHAPTEE V. 



ON IDEAS. 



Man is a thinking being, whether he will or no ; all he can do 

 is to turn his thoughts the best way. SIR W. TEMPLE. 



THE mind operates in scientific research on perceptions or 

 ideas. An idea is also a mental impression ; and, if we adopt 

 the theory of ' unconscious cerebration,' it may or may not 

 be attended by consciousness and perception. It is pro- 

 duced in the cerebrum by nervous force, which is set in 

 motion by various external and internal causes, by external 

 objects or forces acting through the senses, by physical or 



1 Philosophical Magazine, vol. v., 1853, pp. 17-27. 

 D 2 



