CHAPTER XII. 



SENSATION, IDEATION, AND PERCEPTION. 



NEUROLOGY may be advantageously studied by beginning 

 with the investigation of the simplest and earliest forms 

 of the Nervous System, and thence proceeding to examine 

 its more and more complex types. A wholly different 

 order is, however, compulsory, in regard to Psychology. 

 Its ' subjective ' division constitutes for each of us the 

 sphere of positive knowledge in regard to this subject ; 

 while that portion of ' objective ' Psychology having 

 reference to the mental states or processes of our fellow- 

 men has the next greatest amount of certainty for us 

 since the human faculty of Articulate Speech enables 

 us to compare, to some extent, the subjective experiences 

 of other men with our own. 



Objective Psychology, so far as it relates to inferior 

 forms of life, is merely a field for more or less probajble 

 conjecture, in which the basis of certainty diminishes the 

 further we depart from the human type. Knowledge 

 garnered from our own experiences and those of our 

 fellow-creatures affords, as it were, the lamp wherewith we 

 seek to illuminate the dark places of animal Psychology. 

 Hence it is necessary for us in the first place, before 

 attempting to consider the mental processes of lower 

 animals, to look to some of the fundamental facts per- 

 taining to human Psychology. The previous consideration 





