Two extravagances : to exclude Reason, to admit only Reason. — 



Blaise Pascal 



My thanks are due to all those who have helped in the preparation 

 of this book. I am particularly indebted to Michael Oakeshott, 

 of Caius College, Cambridge, to whom must be ascribed many 

 valuable suggestions concerning the general scheme and mar- 

 shalling of the book as a whole. Further I take delight in ac- 

 knowledging the continual inspiration which those who know 

 Sir Frederick Hopkins, F.R.S., are ever accustomed to receive 

 from him. Finally, I thank my wife for all the assistance she 

 has given me. 



J. N. 

 Cambridge y 1925 



