SEC. 5. ON TACTILE PEBCEPTIONS AND JUDGMENTS. 



663. As a means of gaining knowledge of external 

 things the sense of touch ranks next in importance to that 

 of sight. Auditory sensations enter largely and in several ways 

 into our life ; they serve as an important means of communica- 

 tion; together with smell and taste they afford pleasure and 

 guide our acts ; but, as regards our direct knowledge of the ex- 

 ternal world, we learn by means of them very little compared 

 with what we learn by sight and touch. To a certain extent 

 we make use of touch by itself ; we bring the surface of a body 

 into contact with some region of the skin such as the finger, 

 and by the several sensations which we receive either from 

 several points of that region at the same time, or from one or 

 more points in succession, we learn certain characters of the 

 surface, whether for instance it is rough or smooth. We thus 

 also ascertain whether the body be hot or cold ; and we may, 

 within certain limits, form a judgment of the size of the surface 

 by simply estimating the size of the area of our skin with which 

 the body can be in contact at the same time. 



But though we may and do thus base conclusions on tactile 

 perceptions alone, we most frequently employ touch in associa- 

 tion with sight on the one hand and with the muscular sense on 

 the other. 



The ties indeed between touch and the muscular sense are 

 many and close. When we explore the nature of a body by touch 

 we press the skin, of the finger for instance, on the body ; and we 

 do that not merely in order to determine to what extent the tac- 

 tile sensation is increased by the increase of pressure, but also 

 and indeed chiefly to ascertain the amount of resistance to pres- 

 sure which is offered by the body. But that resistance, through 

 which chiefly we judge whether the body be soft or hard, is 

 appreciated not by the tactile but by the muscular sense. 



Or again, placing the finger on the surface of a body, and 

 moving the finger over the surface in such a way that the con- 

 tact, as judged by the pure tactile sensation, remains the same, 

 we find that in one case the movement has been continued in 

 the same plane, whereupon we judge the surface to be flat, that 



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