"It is simplicity itself," he remarked, chuckling at my surprise - "so absurdly simple that an explanation is superfluous; and yet it may serve to define the limits of observation and of deduction.
Observation tells me that you have a little reddish mould adhering to your instep.
Just opposite the Wigmore Street Office they have taken up the pavement and thrown up some earth, which lies in such a way that it is difficult to avoid treading in it in entering.
The earth is of this peculiar reddish tint which is found, as far as I know, nowhere else in the neighbourhood.
So much is observation.
The rest is deduction."
"How, then, did you deduce the telegram?"
"Why, of course I knew that you had not written a letter, since I sat opposite to you all morning.
I see also in your open desk there that you have a sheet of stamps and a thick bundle of postcards.
What could you go into the post-office for, then, but to send a wire? Eliminate all other factors, and the one which remains must be the truth."