All the business of war, and indeed all the business of life, is to endeavour to find out what you don't know by what you do; that's what I called "guessing what was at the other side of the hill."
Source, in full
Remarks to John Wilson Croker and Croker's wife (4 September 1852), quoted in L. J. Jennings (ed.), The Croker Papers: The Correspondence and Diaries of the Late Right Honourable John Wilson Croker, LL.D., F.R.S., Secretary to the Admiralty from 1809 to 1830, Vol. III (1884), p. 276