He knows it's selfish. He shouldn't wish for such a thing.
It's a new feeling, this guilt. He doesn't want to cause any unnecessary – scratch that – any pain at all to John. Dr John Watson who has kept him as safe as he could, as sane as he could, for so long.
He also knows it's hopeless.
It's hopeless to be thinking about this. When it happens it happens. And in his line of work it will most probably be sooner rather than later. But for some reason he can't help it.
He can't help but wish, hope, perhaps even wonder...
Perhaps it will happen the way he knows it will.
Perhaps he will be alone. Without John. With no-one who cares, no-one to try to save him in earnest, no-one to grieve after he's gone.
He can't stop it, he can't stop thinking about it.
Or perhaps it will happen like he hopes it will. He'll be shot, or stabbed. Or maybe even poisoned, then he'll at least have something to do in his last moments. It would be the ultimate challenge to try and deduce the poison burning its way through his body with a fast deteriorating mind. And then he'll be nothing, or there could be an afterlife. It's one thing that Sherlock doesn't know about. Not that he hasn't experimented, bringing himself closer and closer to death, each time trying to catch a glimpse of what might lie ahead for him.
But he does know that he hopes that he won't be alone. It sounds fruitless, redundant and most of all sappy to hope for company whilst one dies. That doesn't stop Sherlock. He wants John there, someone who cares about him enough to try.
John will try, oh he knows that much. John will work in vain over his dying body as he has many times before. It will give neither of them any comfort to know that he tried, tried his hardest to keep Sherlock with him, selfish in his own special way for trying to steal Sherlock back from the brink of the great unknown.
Sherlock doesn't fear death. He doesn't really mind being alone. The thought of being utterly isolated in his final conscious moments on Earth simply creates a wave of panic that douses him from head to toe. His breathing comes in short little pants and he can never seem to get enough oxygen and his legs start shaking violently – John always puts it down to exhaustion.
Currently splayed out on the sofa, stretching his legs out across all three cushions, desperately trying to quell the tremors rocking through the muscles, he caught John's eye over the top of his computer.
"Again?" He sighs before moving his computer to the crowded coffee table and standing stiffly. His movements are jerky, muscles cramped from being inert for a good few hours, and he refuses to meet Sherlock's eye again. Not that had it mattered before. John collapses down onto the sofa and allows Sherlock to rest his head in his lap. Through closed lids Sherlock can see the shadows of John's movements. He doesn't even mind when he brings a hand up to thread gently through his hair.
"Are you ever going to tell me the reason for these panic attacks?"
Sherlock blinks. He hadn't realised John had cottoned on.
"No." You'd laugh at me.
"Why?"
"Because, I don't know why they happen." Because I don't want you to think I'm weak.
John sighs and presses a soft kiss to his forehead, still not meeting his eyes.
"I don't like being lied to, Sherlock."
"I don't want to John, I really don't." For some reason, it saddens Sherlock that he is the only one who knows the ambiguity in his words. Pushing upwards until he is leant rather uncomfortably over him he pulls John's face around.
"Look at me. John. Please, just look at me."
John finally meets his eyes. And Sherlock tries to convey his message wordlessly. He collapses forwards against his chest and his lids slide shut again. Berating himself for not being able to communicate his feelings appropriately, his body shakes violently as another attack breaks over him.
He can't breathe. He feels stripped bare, his mind malfunctioning and pitting itself against his body. He feels as though he were dying.
"Don't you ever leave me." He rasps out to John as he leads him through the worst of it.
As he watches John's eyes fill with enlightenment as realisation dawned on him he nearly cries for the first time in twenty years. John hugs him close, murmurs reassurance in his ear and grips his attention entirely.
Perhaps it will happen the way he hopes it will.
But he sure as hell knows he won't be alone.