28 THE PHILOSOPHY OF BIOLOGY 



intervals may be, we can only think of the point as 

 being at the places 0, A , B, C, D, E, or at /, g, i, j, and so 

 on. We never think of the intervals themselves, and, if 

 all we think about is the position of the point, we do 

 not reallv think of it as in motion at all. We can see 



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it in motion, but we cannot form an intellectual concept 

 of its motion. It is not really necessary that we should 

 in the affairs of everyday life, but for the adequate 

 treatment of problems involving rates of change 

 science had to wait for the invention of the methods of 

 the infinitesimal calculus before this disability of the 

 human mind could be circumvented. 



But the moving point occupies successively a number 

 of different positions in space. If it is a material 

 point that we observe to move from one place to 

 another, we perceive that a certain interval of our 

 duration corresponds with the change of position 

 of the point. Duration was not used up in the 

 occupancy of the different positions 0, A, B, C, D, E, 

 and so on, nor in that of the occupancy of the inde- 

 finitely numerous other positions in which we may 

 place the moving point, but in the intervals themselves. 

 We have said ' duration ' and not " time," using 

 Bergson's term. By duration and time we understand 

 different things. 



Time is, for us, only a series of standard events 

 which punctuate, so to speak, our experienced duration. 

 The unit of time is the sidereal day, that is, the interval 

 of time between two successive transits of a fixed star 

 across the arbitrary meridian. But if we try to con- 

 ceptualise this interval we find that we can do so only 

 by breaking it up into smaller intervals, and this we 

 do by using a pendulum of a certain length which 

 makes a certain number of swings (86,400) during 

 the interval between the two transits of the star. 



