The Language of Music: Sonata No. 1 in g-minor, by Johann Sebastian Bach
This is the piece Sherlock plays when Moriarty comes to visit 221b. The introduction to the fateful scene, one of the most analysed and brilliant scenes in the show.
But it is also the piece that comes to Sherlock’s mind when Eurus asks him to play for her.
Why this choice? He could have played anything but he chooses the piece he played during his fateful encounter with Moriarty, the scene that anticipated the fall after which his life was never going to be the same again. And at this point in TFP he does not even know about a connection between Eurus and Moriarty.
You may know about my Sherlock = Eurus theory. If this is true, Sherlock would be telling himself not to play Bach (the piece associated with Moriarty and all the heartbreak that followed this encounter) but to play his Irene composition (symbolising love and sexual desire). Fine with me.