﻿PREFACE. 
  IX 
  

  

  reasoning 
  of 
  this 
  Essay, 
  whole 
  volumes 
  

   might 
  be 
  written 
  without 
  exhausting 
  

   what 
  is 
  to 
  be 
  said 
  upon 
  them. 
  I 
  shall 
  

   be 
  content, 
  in 
  the 
  meantime, 
  if 
  this 
  

   slight 
  sketch 
  of 
  so 
  great 
  a 
  subject 
  

   should 
  be 
  of 
  any 
  use 
  in 
  directing 
  others 
  

   into 
  some 
  well-defined 
  paths 
  of 
  thought 
  

   and 
  of 
  investigation 
  in 
  regard 
  to 
  it. 
  

  

  London, 
  Dec. 
  9, 
  1868. 
  

  

  