COMPARISON THE BASIS OF CLASSIFICATION. 325 



perception. Simple perception recognises single impres- 

 sions and ideas ; comparison recognises similarity or dif- 

 ference in two or more ideas ; without a plurality of ideas 

 we cannot compare. It is also the simplest form of the 

 testing power of the intellect ; it detects all kinds of simi- 

 larities and differences which are sufficiently strong to 

 arouse perceptive action ; it detects synonyms, identical 

 and equivalent ideas, analogous observations, &c. Abstruse 

 similarities, and identities which are too feeble to arouse 

 perception directly, are enabled to arouse it by means of 

 the power of inference. When we compare, we form a 

 judgment ; and Eeid says, ' The qualities of true or false 

 distinguish judgments from all other acts of mind.' 



Comparison is the basis of classification and generalisa- 

 tion. It is by comparing ideas and things, that we are 

 enabled to form them into distinct groups or classes, each 

 group possessing its own properties or characteristics ; and 

 we thus obtain a means of forming general truths. But 

 the greatest importance of the power consists in its consti- 

 tuting the entire basis of inference ; when we cannot com- 

 pare, we cannot reason. It is by comparing two proposi- 

 tions together, and detecting an identity in them, that we 

 are enabled to infer that to be true of the one which we 

 already know to be true of the other ; it is thus we are 

 enabled to detect abstruse identities. By comparing also 

 abstruse truths together, and detecting further identities, 

 we sometimes perceive still wider truths ; thus by a com- 

 parison of the laws of action of gravity, light, heat, and 

 electric repulsion, we perceive that they all act with an 

 intensity which varies inversely as the square of the dis- 

 tance, and we thus arrive at the more general law of 

 action of central forces, and its agreement with the rela- 

 tions of space. 



Comparison may be either direct or indirect ; we may 



