An Essay on the Signs of Ideas. 248 
faces to the skin ;—by the usual and natural 
action of parts carried to excess, as when 
the muscles are fatigued ;—by the want of 
certain matters necessary to the well-being 
of the system, as when we are hungry, or 
thirsty ;—or, lastly, they either arise sponta- 
neously, or depend upon causes which we 
eannot develope, as when we have the feel- 
ing of itching in the skin, aching in the bones, 
pain in various other parts of the body. 
Such feelings as those which I have just 
described, and which I include under the 
term Simple and original sensations, have, 
each of them, their peculiar seat in pecu- 
liar organs or textures of the body. 
Thus, the eye is so constructed as to re- 
fract all the rays of light proceeding from all 
the points of a luminous surface placed im- 
mediately before it, (provided they fall upon 
the cornea, or transparent coat,) and thereby 
to produce, from every point of the object, 
an impression or sensation of light upon a 
corresponding point at the bottom of the eye. 
No other part of the body is capable of this 
sensation. In like manner the sensation of 
sound is peculiar to the ear, of smell to the 
nose, of taste to the mouth. Thirst is felt 
in the fauces, hunger in the stomach, and 
the sensation of nausea is seated in the same 
