106 HUME 



IV 



During our waking, and many of our sleeping, 

 hours, in fact, the function of ideation is in con- 

 tinual, if not continuous, activity. Trains of 

 thought, as we call them, succeed one another 

 without intermission, even when the starting of 

 new trains by fresh sense-impressions is as far as 

 possible prevented. The rapidity and the intensity 

 of this ideational process are obviously dependent 

 upon physiological conditions. The widest differ- 

 ences in these respects are constitutional in men 

 of different temperaments ; and are observable in 

 oneself, under varying conditions of hunger and 

 repletion, fatigue and freshness, calmness and 

 emotional excitement. The influence of diet on 

 dreams; of stimulants upon the fulness and the 

 velocity of the stream of thought; the delirious 

 phantasms generated by disease, by hashish, or by 

 alcohol ; will occur to every one as examples of the 

 marvellous sensitiveness of the apparatus of idea- 

 tion to purely physical influences. 



The succession of mental states in ideation is 

 not fortuitous, but follows the law of association, 

 which may be stated thus : that every idea tends 

 to be followed by some other idea which is 

 associated with the first, or its impression, by a 

 relation of succession, of contiguity, or of likeness. 



Thus the idea of the word horse just now pre- 

 sented itself to my mind, and was followed in 

 quick succession by the ideas of four legs, hoofs, 

 teeth, rider, saddle, racing, cheating ; all of which 



