PERCEPTION. 365 



adjacent objects. Combined with these ideas are the more 

 immediate perceptions of touch, arising from contact with 

 the skin, and especially with the fingers. All these percep- 

 tions, variously modified, make us acquainted with those 

 mechanical properties of bodies, which have been regarded 

 by many as primary or essential qualities. The perceptions 

 derived from the other senses can only add to the former 

 the ideas of partial, or secondary qualities, such as tempera- 

 ture, the peculiar actions which produce taste and smell, the 

 sounds conveyed from certain bodies, and, lastly, their visi- 

 ble appearances. 



The picture formed on the retina by the refracting power 

 of the humours of the eye, is the source of all the perceptions 

 which belong to the sense of vision: but the visible appear- 

 ances which these pictures immediately suggest, when taken 

 by themselves, could have given us no notion of the situa- 

 tion, distances, or magnitudes of the objects they represent; 

 and it is altogether from the experience acquired by the ex- 

 ercise of other senses that we learn the relation which these 

 appearances have with those objects. In process of time the 

 former become the signs and symbols of the latter; while ab- 

 stractedly, and without such reference, they have no meaning. 

 The knowledge of these relations is acquired by a process 

 exactly analogous to that by which we learn a new language. 

 On hearing a certain sound in constant conjunction with a 

 certain idea, the two become inseparably associated together 

 in our minds; so that on hearing the name, the correspond- 

 ing idea immediately presents itself. In like manner, the 

 visible appearance of an object is the sign, which instantly 

 impresses us with ideas of the presence, distance, situation, 

 form, and dimensions of the body, that gave rise to it. This 

 association is, in man at least, not original, but acquired. The 

 objects of sight and touch, as Bishop Berkeley has justly ob- 

 served, constitute two worlds, which although they have a 

 very important correspondence and connexion, yet bear no 

 sort of resemblance to one another. The tangible world has 

 three dimensions, namely, length, breadth, and thickness; the 



