"You see, but you do not observe."This quote is very important because it's the key to how Holmes constantly amazes Watson by making what he calls deductions (actually inductions) from "the observance of trifles."
-- Sherlock Holmes, "A Scandal in Bohemia"
In a sense, the quote applies to all detective story readers. We don't literally "see" what the detective sees, but a fair-play mystery writer will give us all the clues and yet we usually miss their significance.
For example, in one of Agatha Christie's Poirot novels there are two characters who are related to each other. That relationship is crucial to the solution of the mystery. We can't see the physical resemblance, but Christie has someone mistake one character for the other at a distance -- thereby implying the resemblance.
Most of the time, Sir Arthur Conan Doyle played fair with the reader -- certainly more than almost any other Victorian mystery writer. We are allowed to "see" the dummy bell-rope and the open transom that are so important to the solution of "A Speckled Band," for example.
But there are times that we don't get to see what Holmes does, and therefore don't get the chance to observe or draw conclusions. In "The Adventure of the Red-Headed League," Holmes tells Watson (and us) that he was looking at Vincent Spaulding's knees -- but he doesn't say what he saw there!
What's your favorite Sherlockian quotation?