3o8 SCIENCE PROGRESS 



of a single whole. The result of the connection is that, while its traces last, 

 a repetition of one of the stimuli will liberate energy along the paths of both 

 reactions, and the mental association will be preserved ; in other words, the 

 reaction to the stimulus which is repeated will include the memory of the 

 unrepcated stimulus by which it was previously accompanied. The oftener 

 the two stimuli occur together, the more intimate will be the mental associa- 

 tion ; thus the sight of a familiar surface at once brings to mind the idea 

 of what it feels like. 



Again, the response to a stimulus occupies an appreciable interval of 

 time, during the whole of which the maximum reduction of resistance in its 

 track is maintained. If, then, a second stimulus should act before the re- 

 action to the former one is completed, the reduction of resistance due to 

 the first stimulus may allow energy liberated by the second to accompany 

 the uncompleted discharge, making thereby a connection in consciousness 

 between the two reactions. After the connection has been made, a repeated 

 discharge along the track of the first reaction might again carry with it 

 some discharge from the track of the second, and thus the former would 

 revive the memory of the latter. A repetition of the second discharge does 

 not so easily bring back the memory of the first ; for instance, it is much 

 easier to recall the spelling of a word than to remember how the letters come 

 in the reverse order. 



Since nervous connections are part of, and persist together with, the 

 tracks connected, any one track may be linked up with any other ; for if 

 track A be connected on one occasion with track B, and B on another 

 occasion with C, then a link is made between A and C ; apparently the 

 association between them may afterwards cease to be dependent on B. As 

 experience widens, the choice among the possible linesof mental association thus 

 becomes practically unlimited ; in thought that is concerned with knowing, 

 the selection of a particular line takes the form of framing an hypothesis. 



A sensory nerve is directly connected with two distinct parts of the 

 brain, the thalamus and the cortex. There is reason to think that the 

 mental associations here attributed to connections among the different 

 nerve-tracks are effected mainly, if not solely, through the cortex, while 

 the thalamus does little else than receive the immediate sense impressions 

 from the stimuli. The simultaneous reactions of these two parts of the 

 brain are intimately blended in consciousness, but may be mentally dis- 

 criminated by analysis. 



This points to the solution of the dilemma stated by Hume as follows : 

 " There are two principles which I cannot render consistent, neither is it in 

 my power to renounce either of them, viz. : that all our distinct perceptions 

 are distinct existences, and that the mind never perceives any real relation 

 among distinct existences." (Essay 39, § 7.) 



Here the " distinct existences " are evidently the immediate sensory 

 impressions — the reactions of the thalamus to stimuli — analytically distin- 

 guished from the concomitant response of the rest of the system. The 

 " real connection " is therefore that supplied by the nature of the nervous 

 system, through which the ideas arising from stimuli are presented, not in 

 isolation, but combined with reactions to other simultaneous stimuli and 

 with memories of previous experiences, all blended into an integral whole. 

 On meeting a friend, how little attention do we pay to the sensuous impres- 

 sions — the response to the visual stimuli of his appearance — in comparison 

 with the complex of sensation and memory which is recognition ! The analysis 

 by which the elements originally blended in consciousness are mentally 

 separated is effected by selecting for attention first one, and then another, 

 feature of the complex reaction. 



Experience of the capacity for attention suggests that the resistances 

 determining the course of the discharge of energy along particular nerve- 



