7« cosine PHILOSOFIJY. [pt. ii. 



the total science — parts furtlier specialized hj the intro- 

 duction of additional factors. The astronomy of the solar 

 system is a specialized part of that general astronomy which 

 includes our whole sidereal system ; and becomes specialized 

 by taking into account the revolutions and rotations of 

 planets and satellites. Geology is a specialized part of this 

 special astronomy ; and becomes specialized by joining with 

 the effects of the earth's molar motions, the effects of con- 

 tinuous decrease in its internal molecular motion, and the 

 effects of the molecular motion radiated from the sun. Bio- 

 logy is a specialized part of geology, dealing with peculiar 

 aggregates of peculiar chemical compounds formed of the 

 earth's superficial elements — aggregates which, while exposed 

 to these same general forces molar and molecular, also exert 

 certain general actions and reactions on one another. And 

 psychology is a specialized part of biology, limited in its 

 application to a higher division of these peculiar aggregates, 

 and occupying itself exclusively with those special actions 

 and reactions which they display, from instant to instant, in 

 their converse with the special objects, animate and inani- 

 mate, amid which they move." ^ 



This last point is one which requires further illustration. 

 Concisely expressed, it amounts to this — that psychology is 

 distinguished by dealing in a particular way with the rela- 

 tions between the organism and its environment, A few 

 illustrations will render this perfectly intelligible ; will show 

 as that mere nervous physiology is not, and never can be, 

 psychology. 



Nervous physiology treats of relations subsisting within 

 the organism. It explains how waves of molecular motion, 

 Bet up in a nerve-centre and transmitted along a nerve-axis^ 

 cause contraction in the fibres of a muscle, or secretion in a 

 gland, or molecular rearrangement in the substance of the 

 tissues, or sets up a new molecular undulation in some othei 

 ^ Spencer, Priticiplea of Psychology, vol. i pp. 137, 133. 



