SECT. XVII. 3. 6. OF MOTIONS. 151 



ly pafled over the lips, or by gently vellicating the foles of their 

 feet, laughter is mod vehemently excited ; 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 dimimfh the fenfa- 

 tion, as there would be, if a child mould endeavour to tickle 

 himfelf. See Sett XXXIV. i. 4. 



6. And laftly the motions excited by the junftion of volun- 

 tary exertion with irritation are performed with more energy, 

 than thofe by irritation fingly ; as when we liften to fmall 

 noiles, as to the ticking of a watch in the night, we perceive 

 the mod 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 fpeftra of objects, which remain in the eye, when- 

 ever we pleafe to exert our voluntary power in aid of thofe weak 

 adtionsof the retina, or of the auditory nerve. 



7. The temporary catenations of ideas, which are caufed by 

 the fenfations of pleafure or pain, are eafily diflevered either by 

 irritations, as when a fudden noife difturbs a day-dream ; or by 

 the power of volition, as when we awake from ileep. Hence in 

 our waking hours, whenever an idea occurs, which is incongru- 

 ous to our former experience, we inftantly cliiTever the train of 

 imagination by the power of volition, and compare the incon- 

 gruous idea with our previous knowledge of nature, and rejeft 

 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 effefts 

 in preferving the congruity of our ideas, and bears the fame re- 

 lation to the fenforial power of volition, that irritative ideas, of 

 which we are inconfcious except by their effefts, do to the fenfo- 

 rial power of irritation ; as the former is produced by volition 

 without our attention to it, and the latter by irritation without 

 our attention to them. 



If on the other hand a train of imagination or of voluntary 

 ideas are excited with great energy, and paffing on with great 

 vivacity, and become difTevered by fome violent itimulus, as the 

 difcharge of a piftol near one's ear, another circumftance takes 

 place, which is termed SURPRISE ; which by exciting violent 

 irritation, and violent ienfation, employs for a time the whole 

 fenforial energy, and thus diffevers the pafling trains of ideas, 

 before the power of volition has time to compare them with the 

 ufual phenomena of nature. In this cafe fear is generally the 

 companion of furprife, and adds to our embarraffment, as every 



one 



