SICT. XVII. 3. CATENATION OF MOTIONS. 233 



foles of their feet, laughter is moft vehemently ex- 

 cited ; though they can ftimulate thefe parts with 

 their own fingers unmoved. Here the pleafurable 

 idea of playfulnefs coincides with the vellication ; 

 and there is no voluntary exertion ufed to diminifh 

 the fenfation, as there would be, if s. child fhould 

 endeavour to tickle himfelf. See Seel:. XXXIV* 



i. 4- 



6. And lafHy, the motions excited by the junc- 

 tion of voluntary exertion with irritation are per- 

 formed with more energy, than thofe by irritation 

 fingly ; as when we liften to fmall noifes, as to the 

 ticking of a watch in the night, we perceive the 

 moft weak founds, that are at other times unheeded. 

 So when we attend to the irritative ideas of found in 

 our ears, which are generally not attended to, we 

 can hear them ; and can fee the fpeclra of objects, 

 which remain in the eye, whenever we pleafe to ex- 

 ert our voluntary power in aid of thofe weak actions 

 pf the retina, or of the auditory nerve. 



7. The temporary catenations of ideas, which 

 are caufed by the fenfations of pleafnre or pain, are 

 eafily diflevered either by irritations, as when a fud- 

 den noife difturbs a day-dream ; or by the power of 

 volition, as when we awaken from fleep. Hence in 

 our waking hours, whenever an idea occurs, which 

 is incongruous to our former experience, we in- 

 ftantly diflever the train of imagination by the 

 power of volition, and compare the incongruous idea 

 with our previous knowledge of nature, and reject 

 it. This operation of the mind has not yet acquired 

 a fpecific name, though it is exerted every minute 

 of our waking hours ; unlefs it may be termed 

 INTUITIVE ANALOGY. It is an aft of reafoning of 

 which we- are unconfcious except from its effects in 

 preferving the congruity of our ideas, and bears the 

 fame relation to the fenforial power of volition, that 

 irritative ideas, of which we are unconfcious except 



by 



