78 COSMIO PHILOSOPHY. [ft. u. 



explained "by symbols. Suppose that A and B are two re- 

 lated manifestations in the environment — say, the colour and 

 taste of a friut ; then, so long as we contemplate their rela- 

 tion by itself, or as associated with other external phenomena, 

 we are occupied with a portion of physical science. Now 

 suppose that X and T are the sensations produced in the 

 organism by this peculiar light which the fruit reflects, and 

 by the chemical action of its juice on the palate ; then, so 

 long as we study the action of the light on the retina and 

 optic centres, and consider how the juice sets up in other 

 centres a nervous change known as sweetness, we are occu- 

 pied with facts belonging to the science of physiology. But 

 we pass into the domain of psychology the moment we 

 inquire how there comes to exist within the organism a rela- 

 tion between x and Y that in some way or other corresponds 

 to the relation between a and B. Psychology is exclusively 

 concerned with this connection between ab and x Y ; it has 

 to investigate its nature, its origin, and its meaning."* 



It is true, as the last chapter showed us, that biology also 

 presupposes a reference to phenomena outside the organism, 

 the very definition of Life being " the continuous adjustment 

 of internal relations to external relations"; so that Mind here 

 appears to be but the highest form of Life. We see here the 

 difficulty of sharply demarcating adjacent provinces of na- 

 ture. Nevertheless there is a broad distinction, though not 

 a sharp one. Exclude from biological problems all those 

 adjustments which constitute mental reaction upon the en- 

 vironment, and the only external factors remaining are those 

 general conditions of temperature, moisture, food and the 

 like, which are taken for granted once for all. While in each 

 special problem of psychology, the relation between internal 

 and external relations is the main subject of inquiry ; on the 

 other hand in special problems of biology, the relation be- 

 tween the internal processes and these general externa] 

 * Spencer, Principles of Psychology, vol. i. f. 132. 



