76 
Proceedings of Royal Society of Edinburgh. [sess. 
this implies intelligent discrimination, for “ arrival 55 and “ disappear- 
ance ” are conceptions belonging to a self-conscious being. A series 
is orderly succession, known and contemplated as such. This know- 
ledge organism cannot supply. In the history of three successive 
impressions, No. 1 is displaced to make room for No. 2, and this in 
turn disappears that No. 3 may appear. Impression occurs, and the 
occurrence ends. 
1. (Sensation). 2.(Sensation). 3. Sensation. 
Environment. 
If now’ we include motor activity with sensibility, so making 
account of both sides of the sensori-motor system, a similar chasm 
seems to separate functional action from rational activity. The 
movement of the foot and the utterance of a word are actions 
widely apart. To express thought you must have thought to 
express ; whereas, to move a muscle, no more is required than a 
sensori-motor system, stimulated by contact. The problem is how 
to account scientifically, first , for action provided for by nerve 
stimuli ; and, second , for action provided for by intelligence as its 
necessary condition. We seem hardly ready even yet for grappling 
effectively with this problem. A moderate selection of passages 
from scientific w T orks of the day would demonstrate that we have 
not reached a recognised technical definition of “ voluntary action.” 
Perhaps this can be accounted for in a satisfactory way by the 
present position of science; but as long as this want of exact definition 
continues, the leading problem of our age must be obscure and ill 
understood. What we need is, to define wdth scientific precision 
the difference between rational conduct and motor activity, — the 
contrast which the popular mind sees between movement and 
conduct, as when w 7 e speak of a man’s gait as peculiar, but say of 
his “conduct” that it is wrong. An immense distance separates 
these two. Within this space lies some of the most serious per- 
plexities the hypothesis of evolution has to encounter. What is yet 
to be explained is that which Aristotle signalised as “deliberate pre- 
