"When You Have Eliminated The Impossible, Whatever Remains, However Improbable, Must Be The Truth"
Context: While working on the case of the mysterious disappearance of the father of Miss Mary Morstan, Sherlock Holmes explains and demonstrates to Dr. Watson his method of observation and deduction. As the case develops, Thaddeus Sholto confesses that his father, John Sholto, a good friend to Mary's father, Captain Morstan, was involved in the disappearance and accidental death of the latter. A valuable treasure left by John Sholto on his death had just turned up and was to be shared with Mary Morstan. Thaddeus takes the interested parties to his brother's house where the treasure is supposedly being kept, but on arriving they find that the brother has been murdered. Sherlock Holmes learns that two people had been in the room with the murdered man: one had lowered a rope to allow the second to enter through the window. At this point Dr. Watson cannot understand how the first entered the room. He asks Holmes for the solution, and the detective replies:
"You will not apply my precept," he said, shaking his head. "How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?" . . .