John didn’t use his magic.
Sherlock didn't have magic, which was one of the things that made his skills of deduction so incredibly amazing. He did something that no one else did, and he did it with the just the same abilities that everyone else had. It was amazing.
In contrast, John’s magic was generally useless and he tried his best to avoid using it. As a rule, magicians routinely used magic whenever it would be of use. Most magicians tailored their whole lives around whatever particular magic skill they were born with.
But John didn’t use his magic at all.
“It’s ludicrous,” Sherlock had said. “You have the magic, and whatever skill it gives you, so why not use it?”
John had opened his mouth to say something or another, but Sherlock had cut him off.
“No, no, don’t tell me. This is too interesting for spoilers. Some people don’t appreciate the power imbalance that magic skills give, but you’re a doctor and a soldier: you aren’t scared of having power over other people. And not using it is like voluntary amputation. And despite that psychosomatic limp, you don’t want part of yourself to be gone.”
“That is a terrible metaphor,” John had interrupted. He was happy to let Sherlock toy with various theories, but there were still lines. Anyway, it wasn’t like voluntary amputation at all, since refraining from using his magic wasn’t the same as getting rid of it, more’s the pity. It was more like keeping his gun in a locked drawer: he still had it, he was just choosing not to use it.
Sherlock ignored him. “The average person uses their magic three times a day, counting intentional use when it comes to steady-state powers such as superior strength or knowledge. It’s only the people with rarer and more powerful magics who refrain from flaunting their abilities,” Sherlock mused. And then suddenly, “You’re a miracle worker!”
“Hmm?” John said. He had put down his book to watch Sherlock as he declared his deductive process. John loved watching Sherlock deduct. It was always so fascinating.
“More powerful magicians tend not to advertise their abilities as much, but are more effected by it. The more powerful the magic, the more likely the person is to form their life around the ability.
“You’re a doctor and a soldier. Similarities: Having power of life and death. That falls within the miracle worker range of magic. Can you raise the dead?”
John blinked a few times at the abrupt question. But then, “You are absolutely amazing. But statistical analysis of negative results is not particularly reliable. And no. I can’t raise the dead.”
“But you are a miracle worker, right?” It was the highest known level of magic. Miracle workers performed miracles: created events that contradicted the laws of nature and affected people other than the magician using the magic. Most magic abilities only affected the possessor. Miracle workers could change the people around them.
“No, I’m not a miracle worker,” John said. “And you are just guessing.”
“Hmph. It wasn’t guessing,” Sherlock insisted rather sulkily. “It was testing a hypothesis.”
“Ah, guessing and hoping I’d confirm it?” John shook his head in mock sympathy. “Still guessing.”
“Oh, piss of.” But Sherlock was grinning as he said it. “So, what is your ability?”
“What happened to no spoilers?” John said, mostly to give himself time to consider how he wanted to answer. Party just to annoy Sherlock, though.
“If you never use it, then there’s no way to deduct what it is. You’re obviously a magician, but what’s the power do?”
“Since I don’t use it, it doesn’t do anything much at all.”
“You’re being ridiculous. Of course you’ve used it. Children with magic talents will automatically use them. Knowledge of how the magic works is inherent in the ability itself.”
“I didn’t say I’ve never used it, I said that I don’t use it. I used it as a kid, I decided magic wasn’t for me, and I don’t anymore.”
“That’s just… no one does that! Do you understand? No one! No one just decides not to use their magic!”
“Except you! Why?”
John had returned to reading his book. Sherlock continued to stare intently at him, but he was largely used to that.
“You’re not going to tell me, are you?”
“No, I’m not.”
“In fact, you’ve never told anyone. That's one of the reasons why your relationship with your sister is so fraught. She can tell when people are lying to her. It's helped her be extremely successful as a lawyer and extremely unsuccessful in any personal relationship. Did you lie to her about your magic?"
"Of course not. No point."
"Hmm. Just didn’t tell her anything. Like you’re not telling me. I'll find out eventually."
"I doubt it."
John occasionally saw Sherlock stare at him consideringly after that, especially when magic came up, but Sherlock hadn’t guessed or deduced the truth before he jumped from that roof.
After all, John thought, it wasn't really something that could be deduced. He never used his magic and even when he did, it didn’t do anything measurable.
He wasn’t a miracle worker, because his magic didn’t make exceptions to the laws of nature. He couldn’t change anything that had happened or fix anything that was broken.
His magic worked entirely within the shadow of a doubt: He never knew, could never know, what had changed. Because nothing was changed, or maybe everything was changed.
He had tried using it as a child and it just made him nervous.
As an adult, he only used it once: when Sherlock threw himself off a building and was pronounced dead at the bottom of it.
It had taken several days of careful thought to decide that maybe there was a shadow of doubt somewhere in this whole mess, someplace for his magic to gain purchase. He was probably crazy, but he had to try.
John knew he had seen Sherlock jump, but then he had tripped. He had seen the bloody body dead on the ground. But there had been a moment in between... John had tripped, there was a bike and a truck, there had been a moment of distraction, a moment outside of his immediate sight and knowledge. A moment where maybe his magic could slip in and remake the world around him. As long as he gave it free rein and didn't worry about the repercussions.
Because that was what kept him from using the magic intentionally before: the side effects that by the very nature of his magic he could never know or judge. The world would change around him, the future certainly but also the present and the past, in order to change to his will. Nothing he personally experienced would change, he would only see what had happened beyond his sight by happenstance after the fact, and he'd never know if it had always been so or only became so due to the magic.
He stood at Sherlock's headstone and decided after so many days that he didn't care.
"There’s just one more thing, one more thing,” John was practically stuttering, forcing himself beyond a point he’d promised himself never to pass. “One more miracle, Sherlock, for me.” He had to do this. He just had to. And damned be the consequences. “Don’t… be… dead."
And the magic reached out and changed the world.
He felt it change and shift and morph around him.
And he was still there, still standing on his friend's grave, still going back to an empty apartment, still stressed from so many months dealing with paparazzi stalkings. Nothing had changed or would change about his personal experience.
But he knew that somewhere out there Sherlock lived. Had always lived.
Somewhere out there was a Sherlock who had decided that rather than commit suicide, he had to fake committing suicide. Please god, let the faked death have been by Sherlock’s will and not Moriarty hiding Sherlock’s kidnapping. Either way, Moriarty's network had probably become more powerful than it might have been otherwise, in order to make that decision necessary. Mycroft was probably more powerful and more subtle than he might otherwise have been in order to help fake his brother's death. Who knew how many people were dead or never born who might otherwise have lived, none of whom John knew anything about, in order for the world to be a place where Sherlock survived. Maybe this made the world a better place, maybe a more terrible one. The world was likely more rife with conspiracies than it would otherwise have been.
It didn't matter. It couldn’t matter.
The world didn’t change and leave people in awe of the miracles that had happened: the world always was the way it was. It could never be compared to what it might otherwise have been because there was no one to see and judge between two possibilities.
This was the real reason why John didn’t use the magic he had: because it didn’t change anything. All it did was foster guilt for all the things he could never know.
And yet, he had used it now. And it was worth it. Because he lived in a world in which Sherlock survived.
It changed nothing and yet it changed everything.
And that was why John never used his magic.
And why he used it this once.
Because Sherlock lived!