THE PRINCIPLE OF INDETERMINACY 307 



interference would be a subject for another prediction, 

 but to simplify matters we shall concede it.) It is just 

 this simple prediction which the principle of indeter- 

 minacy expressly forbids. It states that we cannot know 

 accurately both the velocity and position of a particle 

 at the present instant. 



At first sight there seems to be an inconsistency. 

 There is no limit to the accuracy with which we may 

 know the position, provided that we do not want to 

 know the velocity also. Very well; let us make a highly 

 accurate determination of position now, and after 

 waiting a moment make another highly accurate deter- 

 mination of position. Comparing the two accurate 

 positions we compute the accurate velocity — and snap 

 our fingers at the principle of indeterminacy. This 

 velocity, however, is of no use for prediction, because in 

 making the second accurate determination of position 

 we have rough-handled the particle so much that it no 

 longer has the velocity we calculated. // is a purely 

 retrospective velocity. The velocity does not exist in the 

 present tense but in the future perfect; it never exists, 

 it never will exist, but a time may come when it will have 

 existed. There is no room for it in Fig. 4 which contains 

 an Absolute Future and an Absolute Past but not an 

 Absolute Future Perfect. 



The velocity which we attribute to a particle now 

 can be regarded as an anticipation of its future positions. 

 To say that it is unknowable (except with a certain 

 degree of inaccuracy) is to say that the future cannot be 

 anticipated. Immediately the future is accomplished, 

 so that it is no longer an anticipation, the velocity be- 

 comes knowable. 



The classical view that a particle necessarily has a 

 definite (but not necessarily knowable) velocity now, 



