1 86 THE GRAMMAR OF SCIENCE 



§ 13- — Conceptual Time and its Measurement 



Time as a mode of perception is limited, we have seen, 

 to the extent to which sequences of stored sense-impres- 

 sions can be carried back ; it marks that order of percep- 

 tions which is the history of our consciousness. From 

 this it is clear that perceptual time has no future and has 

 no eternity in the past. That consciousness in the future 

 will continue as it has done in the past is a conception, 

 but not a perception. We perceive the past, but we only 

 conceive the future. How, then, we may ask, do we pass 

 from perceptual to conceptual time, from our actual 

 sequences of sense-impressions to a scientific mode of 

 describing and measuring them ? Clearly it would be 

 extremely cumbersome to measure time by a detailed 

 account of the changes in our sense-impressions. Imagine 

 the labour of describing all the stages of consciousness 

 between breakfast and dinner as a means of determining 

 the period which has elapsed between the two meals ! 

 Yet this method of considering time brings out clearly 

 how time is a relative order of sense- impressions, and 

 how there is no such thing as absolute time. Every 

 stage in sense-impression marks in itself an epoch of 

 time, and may form the basis of a measurement of time 

 for an individual. " I am sleepy, it is time to go to bed," 

 says the child ; " I am hungry, it is time to eat," says 

 the savage, and both without thinking of the clock or 

 the sun. Fortunately for us we are not compelled to 

 measure time by a description of the sequence of states of 

 consciousness. There are certain sense-impressions which 

 experience has shown us repeat themselves, and which, 

 on the average, correspond to the same routine of con- 

 sciousness. In the first place, the recurrence of night 

 and day are observed very early in the natural history of 

 man to mark off approximately like sequences of sense- 

 impressions ; a day and night becomes a measure of a 

 certain interval of consciousness. That the same amount 

 of consciousness can, at any rate approximately, be got 

 into each day and night by the normal human being is a 



