Note: This started out as a gif set of the way Holmes talks about ghosts in TAB, because he goes from calmly saying they don’t exist, to saying they do in a sense, to furiously screaming that they do not. It turned into a meta as I tried to work out why his beliefs and mood fluctuate this way (as well as why he goes from saying “we all have pasts” in one scene to “past? what past?” in the next). Spoiler: it has to do with Jim and Redbeard.
On the train: Sherlock asserts there are no ghosts in the paranormal sense. “Save those we make for ourselves” implies a second meaning, and Watson asks for clarity, so this is particularly important. People we wish we hadn’t associated with? Things we wish we hadn’t done? Or alternatively, chances we never took, leaps we didn’t take?
Outside the manor: Holmes defines his second meaning of a ghost as “a past,” which is hardly less vague. “The shadows that define our every sunny day” is such a beautiful line. The word “define” is especially interesting. You can’t have shadows without light. A kind of Buddhist thought…suffering is a part of life (and the root of suffering is attachment, an inability to let go). At any rate, the important thing here is that Holmes says “we all have a past.” Meaning Sherlock has “a past.” He has admitted it.
In the greenhouse: Watson was curious about that whole “ghosts we make for ourselves” line on the train, and apparently concluded Holmes meant something along the lines of romantic regrets, which is why he now presents himself a “brain with no heart.” Watson pulls at this thread in the greenhouse because he’s been puzzling over since the train ride (and of course, this is really Sherlock pulling at the thread in his own mind, exploring his psyche through Watson). And Holmes suddenly plays dumb. “A past?” As if he hadn’t just said they all had one just hours ago! Sure enough, Watson defines “a past” as “romantic entanglements” (which Holmes acknowledges). He doesn’t want to talk to Watson about this (and yet he does, because this is all in his head.) He’s impatient to be attacked by one of those ghosts he doesn’t believe in. He briefly hears Redbeard, then the ghost he’s been waiting for finally arrives.
In the manor: Holmes screams at Watson for insisting he saw a ghost. This type of anger directed at John is extremely rare. Holmes is not talking about ghosts, he’s talking about “ghosts,” aka “a past.” He’s just given Watson the answer to his question from the greenhouse. Sherlock has no past, if past is (by Watson’s definition) a history of “romantic entanglements.” Watson doesn’t seem hurt by the outburst, just surprised…and sympathetic. “What happened?” he asks gently.
What did happen?