Sect. XVII. 3,6, OF MOTIONS. i 5 i 



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 diminiih the fenfa- 

 lion, as there would be, if a child ihould endeavour to tickle 

 himfelf. See Sect, XXXIV. 1. 4. 



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

 tary exertion with irritation are performed with more energy, 

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

 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 fpeefcra of objects, which remain in the eye, when- 

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

 actions of 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 difTevered either by 

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

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

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

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

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

 gruous 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 act of 

 reafoning of which we are unconfeious except from its effects 

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

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

 which we are inconfeious except by their effects, do to the fenfo- 

 rial power of irritation 5 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 cf voluntary 

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

 vivacity, and become difTevered by fome violent ilimulus, 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 fenfation, employs for a time the whole 

 fenforial energy, and thus dilfevers the paffing trains cf ideas, 

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

 ufual phenomena of nature. In this cafe fear is generally 

 companion of furprife, and adds tc - • ijarraflme.ntj as every 



