438 PHYSIOLOGY CHAP. 



according as they are examined by internal or external observation, 

 are presented to us with totally different characteristics, but 

 which are reciprocally, in Fechner's words, interdependent like 

 the two .surfaces, the concave and the convex, of a sphere ; which 

 consist in material changes and correlated psychical alterations, 

 corresponding to the objective and subjective sides of "a unity 

 with two faces " (Bain) ; which appear introspectively as states of 

 consciousness, extrospectively as excitations or states of nervous 

 activity. 



In the preceding chapters we have especially considered the 

 objective side that is, the physiology of neural activity and 

 were we now to attempt any adequate analytical examination of 

 the subjective side, that is of mental activity, we should trespass on 

 psychology and be compelled to renounce the experimental method 

 and to adopt that of introspection, by which alone it is possible to 

 analyse the highest and most complex mental processes. But 

 intermediate between physiology and classical psychology there 

 has recently been developed a new branch -of science, the so-called 

 psycho-physical or physiological psychology, which has abandoned 

 metaphysical speculations about mind, the nature of the soul, and 

 the higher mental operations, and is occupied with the study of 

 elementary psychical phenomena, though it does not confine itself 

 to the consideration of these, but endeavours to evoke them 

 under special pre-determined conditions. The experimental 

 method as applied to the study of mental facts permits us to 

 analyse their constituent elements more readily and to determine 

 their causations and variations under different conditions. We 

 have already applied this purely physiological method in the 

 study of the nervous system and special senses, to bring out the 

 connection between mental phenomena and their somatic basis, 

 to analyse the varied modality and quality of sensations in 

 temporal sequence and spatial coincidence, to measure the 

 duration of elementary psychical processes, and so on. But to a 

 certain extent this involves the analytical study of other more 

 obscure and complex phenomena, associated with different degrees 

 or forms of consciousness, with the processes of attention and 

 memory, and with the reciprocal action of sensations, i.e. the 

 manner in which the simplest mental states influence each other, 

 and are reciprocally evoked, associated, and inhibited. 



In order not to invade the field of pure psychology, we must 

 confine ourselves to the most accessible functions of mental 

 activity in so far as they are connected with neural activity, 

 keeping always to positive knowledge, without attempting any 

 solution of the higher and more abstract metaphysical problems. 



I. Any one who undertakes the study of psycho-physical 

 phenomena observes as one of the cardinal facts that not all 

 stimuli, or states of neural activity, cross the threshold of 



