222 THE NEW QUANTUM THEORY 



known, become connected to our experience and are duly 

 named. But to leave a lot of unattached labels floating 

 in the as yet undifferentiated unknown in the hope that 

 they may come in useful later on, is no particular sign 

 of prescience and is not helpful to science. From this 

 point of view we assert that the description of the posi- 

 tion and velocity of an electron beyond a limited num- 

 ber of places of decimals is an attempt to describe some- 

 thing that does not exist; although curiously enough the 

 description of position or of velocity if it had stood alone 

 might have been allowable. 



Ever since Einstein's theory showed the importance 

 of securing that the physical quantities which we talk 

 about are actually connected to our experience, we have 

 been on our guard to some extent against meaningless 

 terms. Thus distance is defined by certain operations of 

 measurement and not with reference to nonsensical con- 

 ceptions such as the "amount of emptiness" between 

 two points. The minute distances referred to in atomic 

 physics naturally aroused some suspicion, since it is not 

 always easy to say how the postulated measurements 

 could be imagined to be carried out. I would not like 

 to assert that this point has been cleared up; but at any 

 rate it did not seem possible to make a clean sweep of 

 all minute distances, because cases could be cited in which 

 there seemed no natural limit to the accuracy of deter- 

 mination of position. Similarly there are ways of 

 determining momentum apparently unlimited in accuracy. 

 What escaped notice was that the two measurements 

 interfere with one another in a systematic way, so that 

 the combination of position with momentum, legitimate 

 on the large scale, becomes indefinable on the small 

 scale. The principle of indeterminacy is scientifically 

 stated as follows: if q is a co-ordinate and p the corre- 



