456 THE SKNSORIAL FUNCTIONS. 



his application, can withstand the influence of 

 impressions on his external senses ; for, if suffi- 

 ciently reiterated or intense, they will always have 

 power, if not to engross his whole attention, at least 

 to interrupt the current of his thoughts, and direct 

 them into other channels. Nor is it necessary for 

 producing this effect, that cannon should thunder 

 in his ears; the mere rattling of a window, or the 

 creaking of a hinge will often be sufficient to dis- 

 turb his philosophical meditations, 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 distin- 

 guishing 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 phy- 

 sical changes in the brain accompany the mental 

 processes of thought, we are in utter ignorance 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, therefore, 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 



