52 INTRODUCTION. 



Impressions are left upon the medullary masses 

 by the modifications they have thus experienced, 

 which are capable of reproduction, and thus recall to 

 the mind its former images and ideas. This it is 

 which constitutes memory, a faculty therefore de- 

 pending upon organization, and varying consider- 

 ably, according to the age and health of the indivi- 

 dual. 



tion is called a sensation ; and the second is a perception, 

 of something external to ourselves, which we assign as the 

 cause of the sensation experienced ; and this is perception pro- 

 perly so called. The perception of the external object generally 

 follows the sensation so instantaneously as to engross the whole 

 attention of the mind, except in cases where the latter is remark- 

 ably acute. Instantaneous, however, as it is, it. is by no means a 

 simple operation, and, in the earlier stages of our existence, is 

 progressive and slow. Our knowledge of any external object being 

 gained through the medium of different senses, the idea of that 

 object must of necessity be complex, though apparently uncom- 

 pounded. One quality of an object is acquired by one sense, and 

 another by a different sense ; yet when the same object is pre- 

 sented afterwards to any one of our senses, we immediately recal 

 to mind all the qualities that we have discovered in it through the 

 medium of the others. This is one of the phenomena of associa- 

 tion. 



The perfection of every animal depends mainly upon the extent, 

 vividness, and accuracy of its perceptions. Though some surpass 

 man in the acuteness of certain of their senses, we have no reason 

 to suppose that any equal him in the comprehensiveness and cor- 

 rectness of his general perceptions, that is, in the result of all the 

 senses combined. The superior acuteness of his percipient faculty 

 raises man infinitely above all other animals, and would still raise 

 him above them even were his external organs or instruments of 

 perception less perfect than they are. As we descend in the scale 

 of creation we find the perceptions of the animal world grow much 

 more circumscribed, and among the lowest tribes we have reason 

 to believe that little more exists than mere sensation. 



