PREFACE 



PREFACE TO THIRD EDITION 



This edition differe only sUghdy from the previous one. The 

 opportunity has been taken to make a few alterations, mostly of 

 a minor nature, and add to the Appendix two good stories iUus- 

 trating the role of chance. 



Cambridge, September 1957. W.I.B.B. 



xu 



