From: Elementary 6.2
Line: SHERLOCK: It was easier to know it than to explain how I knew it. If you were asked to prove that two and two made four, you might find some difficulty. And yet you are quite sure of the fact.
Reference: this quote in the Elementary ep is Sherlock’s response to an incredulous FBI agent (no spoilers–this ep aired recently), and this exact same quote, verbatim, was uttered by Holmes to an incredulous Watson, in one of the earliest moments in the duo’s relationship of detective and record-keeper. This exchange took place in the very first Sherlock Holmes story, the novel-length A Study in Scarlet, after Watson couldn’t quite believe how Holmes saw the commissionaire’s situation just by glancing at him out a window.