DECLHSTE OF DETERMINISM — EDDINGTON 143 



To see the change that has occurred, we can consider a recent book 

 which goes as deeply as anyone has yet penetrated into the funda- 

 mental structure of the physical universe, Dirac's Quantum Mechan- 

 ics. I do not know whether Dirac is a determinist or not; quite 

 possibly he believes as firmly as ever in the existence of a scheme of 

 strict causal law. But the significant thing is that in this book he 

 has no occasion to refer to it. In the fullest account of what has yet 

 been ascertained as to the way things work, causal law is not 

 mentioned. 



This is a deliberate change in the aim of theoretical physics. If 

 the older physicist had been asked why he thought that progress 

 consisted in fitting more and more phenomena into a deterministic 

 scheme, his most effective reply would have been " What else is there 

 to do ? " A book such as Dirac's supplies the answer. For the new 

 aim has been extraordinarily fruitful, and phenomena which had 

 hitherto baffled exact mathematical treatment are now calculated 

 and the predictions are verified by experiment. We shall see pres- 

 ently that indeterministic laAv is as useful a basis for practical pre- 

 dictions as deterministic law was. By all practical tests progress 

 along this new branch track must be recognized as a great advance 

 in knowledge. No doubt some will say " Yes, but it is often neces- 

 sary to make a detour in order to get round an obstacle. Presently 

 we shall have passed the obstacle and be able to join the old road 

 again." I should say rather that we are like explorers on whom at 

 last it has dawned that there are other enterprises worth pursuing 

 besides finding the Northwest Passage; and we need not take too 

 seriously the prophecy of the old mariners who regard these enter- 

 prises as a temporary diversion to be followed by a return to the 

 " true aim of geographical exploration." But at the moment I am 

 not concerned with prophecy and counterprophecy ; the important 

 thing is to grasp the facts of the present situation. 



SECONDARY LAW 



Let us first try to see how the new aim of physical science origi- 

 nated. We observe certain regularities in the course of nature and 

 formulate these as laws of nature. Laws maj be stated positively 

 or negatively, " Thou shalt " or " Thou shalt not." For the present 

 purpose it is most convenient to formulate them negatively. Con- 

 sider the following two regularities which occur in our experience: 



(a) We never come across equilateral triangles whose angles are 

 unequal. 



(h) We never come across 13 trumps in our hand at bridge. 



In our ordinary outlook we explain these regularities in funda- 

 mentally different ways. We say that the first occurs because the 



