PERCEPTION 83 



factory test of truth is possible upon its grounds. If one 

 knows the material world only through immaterial ideas 

 and if the truth of an idea consists in its correspondence 

 with an object, then truth can never be ascertained. One 

 can get ideas but one cannot get material objects, hence no 

 comparison can be made. One cannot compare his image 

 of a piece of gold with the piece of gold itself, for he can 

 know the latter only through the intermediary of an image; 

 hence he finds that he can only compare one image with 

 another. The result is that he is isolated within the realm 

 of ideas and can never know an external world. The second 

 inadequacy of the common sense theory is its failure to find 

 a place for thought creations, fictions, constructions, abstrac- 

 tions, and the like, in the cognitive situation. From the 

 point of view of science this is a serious limitation. Without 

 idealizations, isolated systems, serial extensions beyond and 

 interpolations between the elements of the given, science 

 would find it impossible to proceed. In other words, science 

 finds it useful to convey information about something which 

 does exist by talking about something which does not exist. 

 According to the common sense theory this would be impos- 

 sible; all ideas of things which do not exist would be false 

 and could not contribute to knowledge. Common sense 

 denies any creative activity to the knower, since any trans- 

 formation which is due to his activity could only result in 

 error. The perceiver is merely the passive recipient of stimuli, 

 and his efficiency is to be measured in terms of his receptivity. 

 As a result of these inadequacies various attempts have 

 been made in the history of thought to replace the common 

 sense theory of perception by one which allows for the 

 possibility of science. A detailed discussion of these the- 

 ories would be out of place in an introductory book of this 

 character. Two of them may be briefly considered here, 

 viz., positivism 1 and subjectivism. 



1 Positivism is both a theory of perception and a theory of description. It will be 

 considered only in the former sense here. Positivism as a theory of description will be 

 discussed in Chapter VIII. 



