Sherlock's series four finale airs this Sunday 15th January – and the episode, titled The Final Problem, is set to be something pretty special if creators Steven Moffat and Mark Gatiss are to be believed.

 Ahead of the episode, we've had plenty to discuss – not least because of the beguiling twist at the end of episode two last week.

But some fans are delving further back in to Sherlock series history in the hope of coming up with some answers.

And you know what? They may have found something, a fizzing scene featuring Benedict Cumberbatch's Sherlock and Andrew Scott's Moriarty from series two episode three, The Reichenbach Fall.

The interchange ends something like this.

SHERLOCK: What is it all for?

MORIARTY: I want to solve the problem. Our problem. The final problem. 

The Final Problem of course is not just the name of the upcoming episode on BBC1. It's also the name of the Arthur Conan Doyle book where Moriarty and Sherlock apparently meet their fates in a struggle over the Reichenbach Falls.

But in the TV series, it means even more.

Later in the episode above – spoiler warning – Moriarty implies that the "problem" is the problem of "Stayin' Alive" (remember the ringtone?). It was a problem he apparently failed to solve when he blew his brains out on top of the hospital roof.

But – and this has been the question hanging over the series ever since that moment – if Sherlock could fake his own death, why not Moriarty?

So instead of trying to work out exactly who Eurus is from last week's episode, maybe we should prepare ourselves for a real, actual return for Moriarty, who somehow shows he is just as capable of "Stayin' Alive" as Sherlock?

Or perhaps not. Perhaps all the riddles of this series, including the fable of The Appointment in Samarra from episode one,  are designed to remind us that nobody – not Moriarty, not Mary, not even Sherlock himself – can outrun death forever.

Problem? Prepare for the solution: Sherlock series 4 finale airs Sunday 15 January at 9pm on BBC1.