302 MENTAL FACULTIES sec. 



Since all nervous action must have been gradually evolved 

 through relations of the organism to the outer world, the diffi- 

 culties referred to lie in the nature of the case, and can there- 

 fore only be considered as in favour of the justice of the view 

 I advocate. But these difficulties constantly make themselves 

 felt to an uncomfortable degree in the attempt to distinguish 

 between impulse and instinct. Ought we really to describe 

 the sexual impulse as an instinct ? Certainly it has as much 

 right to be so called as the migratory impulse, which appears 

 in spring and autumn, even in birds which have been reared 

 and kept all their lives in cages. The migratory impulse is 

 evidently excited by hereditary changes in the circulation 

 which occur at those times, by local blood pressure, which 

 again influences the nervous system, while the latter, on its 

 side, is excited by this change in the condition of the body to 

 the inherited automatic production of a number of acts which 

 are connected with this change. In principle the same is true 

 of the sexual impulse. 



And yet impulse is not instinct, as has often been hastily 

 assumed. Impulse is merely the urgent desire to get rid 

 of an unsatisfactory condition of the body, to remove an 

 unpleasant feeling. Instinct finds the appropriate means of 

 effecting this : thus e.g. not the sexual impulse, but the pur- 

 poseful "endeavour to satisfy the impulse, is an instinct. But 

 the two things, the impulse, and the endeavour to satisfy it, 

 cannot be separated : thus, to use the case of the migratory 

 impulse again, the physiological condition of the body 

 which produces it, and the expression of the impulse itself, 

 cannot be separated. Similarly, hunger and thirst are so far 

 instincts that they cannot be separated from the instinct of 

 seeking food. Physiology describes both as general feelings : 

 they show themselves as needs felt by the nervous system, 

 caused by an internal condition of the body acting as a 

 stimulus — needs which must so far be called instinctive, that 



