MONITORS 227 



measuring the passage of time seems not unreasonable at first 

 sight, and we can find examples fi-om our own experience. 

 In the first place our own physiological processes give some 

 indication — we do not need a clock to tell us that dinner- 

 time is approaching. On the other hand this is because we 

 are in the habit of eating our meals at approximately constant 

 times day after day; were we, like many wild animals, to 

 feed whenever the chance of getting food occurred our 

 physiology would tell us merely that we were hungry and 

 would give no indication of the time of day. 



Many people are, however, able to wake with great 

 accuracy at an unusual predetermined hour, especially when 

 they have a strong reason for doing so, such as the necessity 

 of catching an early train. This facility is so striking that it 

 may well lead to a belief that we have some internal way of 

 registering the lapse of time ; but what of the person who is 

 so anxious lest he oversleep that he wakes every half-hour or 

 so throughout the night? If he has an internal clock his sub- 

 conscious mind appears to set little store by its accuracy. It 

 is possible that waking at the desired hour is achieved by 

 using external stimuli. If the hour is an unusual one the 

 anxiety not to miss it may cause sleep to be lighter than 

 usual so that external stimuli from the sense organs are not 

 rejected or ignored by the brain. It is not completely impos- 

 sible that we may subconsciously count the ticks of the bed- 

 side clock or watch ; although we may not consciously know 

 the number of quarter-seconds that must go, perhaps we do 

 subconsciously. Other external stimuli may be used, such as 

 the striking of the distant church clock, which normally we 

 do not hear at all. If the use of such external signals be 

 denied there is the alternative that we may similarly count 

 our own heart-beats or respirations, though as their rates are 

 not so constant as the escapement of a clock it is hard to see 

 how they could produce any great accuracy. 



Strong support of the theory that external stimuli are used 

 is given by the way in which all sense of time is quickly lost 

 in their absence. Cave explorers, for example, if by some 

 accident they are deprived of their illumination and have 

 to wait until they are rescued, after a moderate interval 

 have no idea of the hour, or even of the day of the week, 



