512 THE SENSORIAL FUNCTIONS. 



window, or the creaking of a hinge will often 

 be sufficient to disturb his philosophical medi- 

 tations, and dissever the whole chain of his 

 ideas. " Marvel not," says Pascal, " that this 

 profound statesman is just now incapable of 

 reasoning justly ; for behold, a fly is buzzing 

 round his head. If you wish to restore to 

 him the power of correct thinking, and of dis- 

 tinguishing truth from falsehood, you must 

 first chase away the insect, holding in thraldom 

 that exalted reason, and that gigantic intellect, 

 which govern empires and decide the destinies 

 of mankind." 



Although we must necessarily infer, from the 

 evidence furnished by experience, that some 

 physical changes in the brain accompany the 

 mental processes of thought, we are in utter ig- 

 norance of the nature of those actions ; and all 

 our knowledge on this subject is limited to the 

 changes which we are conscious are going on in 

 the mind. It is to these mental changes, there- 

 fore, that our attention is now to be directed. 



In experiencing mere sensations, whatever be 

 their assemblage or order of succession, the mind 

 is wholly passive : on the other hand, the mind 

 is active on all occasions when we combine into 

 one idea sensations of different kinds, (such as 

 those which are derived from each separate 

 sense), when we compare sensations or ideas with 

 one another, when we analyze a compound idea, 



