A/N: Title is from Vincent Starrett's poem "221B." The full line is: "Only those things the heart believes are true."

I.

As a little boy, Sherlock, grieving, terrified, and traumatized, had entered his mind palace—that memory construct that Mycroft had taught him and…Eurus—with one intention: to change his own memory. The Trevor family had once owned a lovely Irish Setter that had been hit by a car and had to be put down. That would be a better story. He simply couldn't remember his dead friend…Victor…drowned Redbeard…any longer.

Sherlock entered his mind palace with the intent to change his memories. He was never the same again.

II.

Ever since he had been put down, Redbeard had lived in Sherlock's mind palace. Whenever his parents were bothering him to eat more, strange worry in their eyes, or Mycroft was telling him again that he, Sherlock, may be stupid, but everyone else was an utter idiot and they didn't need friends, or his violin was, for some reason, only making him sad, he could go find Redbeard.

Here boy! Come on! Good boy, Redbeard, burying his face in the dog's fur. I miss you. I'm sorry they had to put you down. Such a good boy.

And if Redbeard always stayed in that one hallway in the mind palace, distracting Sherlock before he could try to go through the doorway at the far end, well, it was Sherlock's own mind that had constructed the distraction. It was natural that he would be unsuspicious of it.

III.

The first time Sherlock's mind used someone else's voice he was practicing deducing—another thing Mycroft had taught him. The new mailman's tan—did he used to work outdoors, or had he just come back from vacation? The man wore long sleeves—where did his tan lines stop?

Don't be stupid, sneered the mind palace construct of his older brother. His tan lines don't matter.

He's not stupid! He's going to be good at this—better than Mycroft! Better than…anyone!

His eyes, Sherlock! No one gets such a habitual squint from a mere vacation. Therefore…

Of course! He scampered off to tell Mummy, who didn't seem to care much. Then Mycroft came into the kitchen.

"Did you observe the new postman, Sherlock? He is a former outdoor laborer. You can tell by…"

"His tan and his squint, yes, Mycroft, you already told me."

"Go play deductions somewhere else, dears. I'm trying to concentrate."

Sherlock opened his mouth to protest, but Mycroft dragged him out of the room by his arm.

"Told you? I didn't tell you anything about the postman."

Sherlock rolled his eyes. "Not you you, but you. Up here." He tapped his temple.

"I'm in your mind palace? Why would you put people in your mind palace?"

"I didn't put you there. You just showed up and started telling me how to deduce!"

"The mind palace is a memory technique, Sherlock. It can only tell you things you already know." Mycroft paused, then crossed his arms and raised his eyebrows at his little brother. "Your mind giving that information in other people's voices can only be emotional bias. If you want to be better, maybe try to get rid of the fly in the ointment."

Sherlock sulked, and tried to banish mind palace Mycroft. He wouldn't banish.

IV.

As he grew older and met more people, Sherlock's mind was regularly invaded by a rotating cast of characters. His parents featured periodically but not prominently, reminding him of manners or chores, but the more he ignored them the less they showed up. Occasionally he gave in and responded to his mother's inner promptings by saying "please" or "thank you," but it hardly seemed the social lubricant she claimed. Maybe his tone of voice was wrong? He rarely heeded the prompts, anyway.

If he had distracted himself during class, a teacher might show up later to recount the lesson in his head. If it was particularly interesting, he might file the knowledge away. Information was information, after all, even if most of the teachers were idiots. (If the lecture was particularly boring, he would just shut the teacher in one of his mind palace rooms and go play with Redbeard instead.)

His classmates sometimes tried to waylay him in his mind palace, too, but Mycroft revealed one of his few uses by always showing up to remind him that caring was not an advantage. So Sherlock practiced retaliatory and eventually preemptive insults against the mind palace classmates. They were often useful later in the real world.

V.

For some reason, Sherlock could not get Carl Powers' unknown murderer out of his head. The murderer had no form, no voice except a breathy whisper hissing at him from the dark corners of his mind palace.

He played his violin to shut it up.

VI.

Slowly, slowly, some of Sherlock's temporary mind palace guests began taking up a more permanent residence.

The widow of the Florida drug dealer whose execution he'd guaranteed began following him around, berating him for making a mess of the files in his mind palace even as she worked to restore order. The detective inspector who'd desperately promised him cases if he got clean was always there to remind him that drugs mean no case work. The mousy but competent St. Bart's pathologist would hover over his shoulder with medical information until he banished her with a wave of his hand. Even the New Scotland Yard sergeant and forensics lead showed up every so often, though like the classmates of his youth he paid them no attention.

Freak! Psychopath!

No. Sentiment was a chemical defect. There was nothing wrong with him.

VII.

John Watson.

A mad, wild chase through London. That…was amazing. The last heady moments of the case, broken by the sharp reverberation of a pistol shot.

Suddenly, Sherlock had a colleague, a partner, a friend. It was glorious. With the ease with which John slipped into his life it was no surprise he also immediately joined the ranks of mind palace residents. He was almost always there. Sometimes he might join Molly in relaying medical information, but more often he provided perspective on cases, or shouted at Sherlock to call for back-up, or gave running commentary on Sherlock's social interactions. Marvelous, he might say, or a bit not good, or any other combination of praise or censure. He was more real, more true to his real world self, than any other denizen of Sherlock's head.

Of course, the problem with this was that sometimes Sherlock accidentally responded to mind palace John instead of real-world John. This led to tension between the flatmates.

"Where are we going, Sherlock?"

"The opera, John, I told you yesterday!"

"You most certainly did not."

Or:

"Right. You've been talking to me all morning, when I wasn't actually here. Why do you need me again?"

Sigh. "John…"

He never bothered trying to explain. If even Mycroft didn't understand this particular mind palace phenomenon, how could anyone else?

VIII.

Moriarty. Carl Powers' murderer had finally gained a name.

At first this was exciting. The game is on! Finally, finally, he stopped being bored. Finally, finally, he could hiss right back at the taunting whispers in his mind palace. Finally, finally, someone appreciated how clever he was.

(John appreciated how clever he was. But John, for all that Sherlock cared for him, was ordinary. He could admire Sherlock's mind, but he couldn't match it. Moriarty, it seemed, could.)

Then Moriarty threatened to kill John and the whole beautiful game came crashing down around Sherlock's ears. The name gained a face and a voice at the same time that it lost Sherlock's fascination and acquired only his revulsion. After the pool, when Moriarty showed up in Sherlock's mind palace, he took to locking him into the nearest room and hoping he stayed there. But he always got free, and the hissing continued, now in a lilting Irish voice laced with insanity.

IX.

The Woman, brilliant, alluring, deadly. His fascination was immediate and complete. Like him, like Mycroft, like Moriarty, she saw the world laid out in clear patterns and played its strings like he played his violin. But it was her body, not her mind, that she wielded as a weapon. He was drawn to her, willing her to be as enthralled by him as he was by her. Showing off for her.

Mind palace Mycroft berated Sherlock for that blunder even more than real-world Mycroft.

Love is a dangerous disadvantage.

He would not be on the losing side again.

But he did save her life, whatever Mycroft's minions thought they knew, and she, in turn, kept texting him. He never responded. She kept showing up in his mind palace naked, however, and he didn't know why, nor how to respond. Was there something wrong with him and Mycroft? Love was most definitely not his area.

X.

Then Moriarty returned and a clench of dread settled permanently in Sherlock's gut. In his mind palace, the man followed him around constantly, predicting his real-world counterpart's every move, scoffing at Sherlock's upcoming humiliation. Sherlock kept trying to lock him up but it did no good at all. A very real fear settled across him: the situation was spiraling out of all control, and he couldn't even seem to regulate his own mind.

But he pieced together enough of Moriarty's plan to preempt his worst, last move, planned desperately with Mycroft and Molly, reluctantly enlisted the aid of some of his homeless network, and stubbornly blocked John's upcoming grief from his mind.

Let me help, mind palace John pleaded. I can go with you to destroy Moriarty's network. The two of us against the rest of the world. Sherlock, please!

No! No, it's my fault, it's too dangerous, I won't have you die for me! No!

It was usually at this point that an unseen sniper shot John through the heart and Moriarty appeared behind him, laughing, laughing.

I said I'd burn the heart out of you!

Sherlock shuddered at the unholy laughter and the sight of John's lifeless body. Sometimes his mind was far too good at verisimilitude.

XI.

Sherlock fell, was caught, abandoned all he knew and disappeared into the depths of Moriarty's criminal network, and nothing was ever the same again.

XII.

While Sherlock was away, mind palace Moriarty took to showing up with half his head missing at the most inconvenient times. Once, the distraction nearly got him real-world killed. As soon as he limped to the safe-house and tended his wounds as best as he could, he shut himself up in the mind palace and, in a sequence probably caused as much by blood loss and the morphine he'd injected to battle the pain as his conscious intentions, wrangled Moriarty to submission in a series of increasingly surreal locations and circumstances, from London's back alleys to the criminal hideouts he currently frequented to the slippery stone ledge above the Reichenbach Falls.

Pushing Moriarty into the waterfall was satisfying. Subsequently forcing him into a straitjacket and newly-constructed padded cell in the lowest room of his mind palace was unpleasant but necessary. It also seemed to have finally worked: Moriarty stopped taunting him from corridors when he needed to concentrate.

XIII.

John was as frequent a companion in his mind palace as he had always been, but now his voice had taken on a bitter tinge. It was rare for praise to pass his lips. Show-off, with an exasperated sigh, was more common, or bloody liar, as he constructed another elaborate disguise and wormed his way into another criminal ring. Sometimes even you heartless machine with a scowl on his face and a clenched fist. Sherlock never tried to reason with mind palace John, just worked harder and harder: clearly his mind had gotten something wrong this time. He needed access to the real John to reconfigure his mind palace version correctly. He needed to finish and get back to London.

XIV.

One day, waiting for some targets to take his bait, he had idly checked The Science of Deduction. His brows scrunched disbelievingly: the hit count was through the roof, and his most recent post (which was not recent at all, nor particularly interesting even to him) had an unprecedented number of comments. As he scrolled through them, his disbelief grew.

I believe in Sherlock Holmes!

Sherlock is Alive!

If you're reading this, Mr. Holmes, just know we're all rooting for you! Whatever made you leave, please come back as soon as possible!

Dr. Watson may have forgotten, but we're your fans and we never will!

For a moment his heart stopped in his chest. Would all his plans, all his care, be ruined by these morons who couldn't accept a perfectly believable death? Never mind that it was true; if any of Moriarty's remaining henchmen with their standing orders even suspected that he was alive, John and Mrs. Hudson and Lestrade would be shot before he could even arrange passage back to London…

Calmly, brother, Mycroft interjected smoothly. You're allowing emotion to cloud your judgment. What is the date of the comments?

It was…months ago, for many of them. Almost a year.

You heard from me yesterday that your friends are safe and sound. If the assassins haven't responded yet, how likely are they to do so now?

Not likely. But Sherlock couldn't depend on probabilities. If John were convinced of the conspiracy the assassins would be, too. He clicked quickly to The Personal Blog of Dr. John H. Watson. The second-to-last post was from directly after the Fall, stating merely that yes, Sherlock was dead and no, John didn't believe he was a fraud (so much for Sherlock's very convincing phone call note) and the blog would be shut down for the foreseeable future, please don't bother him.

The top comment on that post said: Dr. Watson, you can't give up hope! We all believe in him, why don't you? You're his best friend! Please, just look at the evidence. It gave a hyperlink to a website apparently titled Sherlock Lives.

But Sherlock sighed in relief, for John's most recent post simply said: You all weren't there. There was no room for doubt. I'm disabling comments on this blog; please stop bothering me. You're only making it worse.

If John didn't believe it, no one else would. His friends were safe.

But his targets still hadn't materialized, so he, curious, clicked on the link to Sherlock Lives. We Believe in Sherlock Holmes! read the byline, and it was mostly impossible theories about how he had could have survived the fall and supposed evidence for his still being alive. There were myriad false sightings and photographs—as though, having gone to all the trouble to fake his death, he would have remained in London watching his friends from the shadows instead of going abroad to take down Moriarty's network!

But he was halfway alarmed and halfway impressed to see that one of the moderators had actually correctly identified two of his recent take-downs. Of course, the post was sandwiched between an idiotic theory that he had been saved by the fictional time-traveling doctor in that mind-numbing sci-fi show John had made him watch and some moron's supposed evidence from his demeanor at the trial that he and Moriarty were partners who had deceived John together. The moderator's name was ImSorrySherlock and, as Sherlock read through the corresponding paragraph on the About the Mods page, he suddenly came to a surprising but incontrovertible conclusion.

That's Anderson.

At that moment his targets finally showed up where they were supposed to, and he shunted everything else to the back of his mind so he could spring the trap.

And if, when Mycroft's deductions and Molly's medical advice weren't quite enough to stop his hand from shaking and convince him to get up, keep moving, and focus on his next target, Anderson showed up to quietly remind him that there was a London that believed in him and would welcome him back unquestioningly, Sherlock merely accepted the encouragement. Sometimes, within the safety of his own mind, he could admit his own weakness. He was discouraged, depressed, strung-out, injured, unsure, and alone. He needed all the help he could get, even from an Anderson conjured up by his own tired mind.

XV.

Returning to London didn't have quite the effect Sherlock expected. His mind palace version of John was, as it turned out, more accurate than his apparently sentiment-corrupted, nostalgia-tinged memories. John was neither pleased to see him, nor interested in how he'd done it, nor congratulatory for his taking down Moriarty's web. He did not, in fact, seem to want to see Sherlock ever again.

Mind palace John called him an idiot for ever thinking things could go back to how they were but kept intruding into his thoughts anyway, however much Sherlock tried to banish him.

XVI.

Mary turned out to be a better friend than he ever would have expected. She liked him. She quickly helped reconcile Sherlock and John, and just as quickly joined John in the mind palace.

Then, "Oh, Sherlock, if you take one more step I swear I will kill you."

No, I won't, mind palace Mary refuted. I'm panicked even if I don't look it, but you're my friend I won't kill you.

"No, Mrs. Watson. You won't," he replied, confident. And his mind, it turned out, was not wrong, but neither was it right. Not kill him, maybe; but definitely hurt him. And he certainly felt like he was dying.

But even when Sherlock was in pain, dying, and unable to form rational thought, his mind knew who could save him: Mycroft, the manipulator; Molly, the confidant; Anderson, the believer; and Moriarty, the incentive. Together, they kept his heart beating and restarted it when it quit.

Mycroft's lifeless mind palace, without almost-autonomous copies of his friends and allies, couldn't have done such a thing.

Perhaps caring was, sometimes, an advantage, after all.

XVII.

With the help of a dangerous cocktail of drugs, Sherlock's mind conjured up not only all his friends but in fact an entire world in order to solve a hundred-year-old crime and determine whether there was any way Moriarty, having blown his brains out, could have come back from the dead.

The answer was a resounding no.

And however interesting it was that, with the proper fuel, his mind palace could become…that…Sherlock rather hoped to never experience such a thing again. It was far, far to close to being trapped in his own mind.

XVIII.

Mary died, and for a while he was numb. Mind palace John could offer no advice on how to help real-world John, and real-world Molly was no more insightful. He couldn't find mind palace Mary, and of course real-world Mary was dead.

Except Mary had been an assassin, competent and brilliant and capable, and she'd left him instructions. "Save John Watson," except he didn't know how, but the amazing woman knew that, too, told him how. He didn't need to watch the DVD more than once; in his mind palace, Mary was now back in her rightful place, giving him instructions on exactly what to do to salvage his wreck of a friendship.

XIX.

Once he was on the drugs in earnest, he avoided going to his mind palace at all, not having forgotten his previous drugs-plus-mind-palace experience. He didn't need Moriarty loose in his head again, even if mind palace John would always be there to help defeat him.

XX.

"I cheated on you, Mary," said John, miserable and ashamed. And Sherlock, who after all knew that merely hearing voices in one's head didn't make one crazy, was perfectly willing to set his friend's mind at ease on that score by replying to mind palace Mary out loud.

(Perhaps, at some point, he could even tell John the whole truth about the mind palace and its inhabitants. Mycroft had never understood how his mind palace could be anything but a visual memory storage bank, but John, just maybe, could.)

XXI.

"We never had a dog," and Sherlock's world would never be the same again. No time, now, to think, only to react: save Eurus from her own horrific type of mind palace that trapped her in its allegory (and hadn't his almost done something similar?); save John from the well that had been the death-trap of his first—and for such a long time, only—friend. For yes, he began to remember now. Even as, in the real world, his feet pounded the slick grass of the Musgrave grounds, in his mind palace he flew past the poor Irish Setter guard dog that had never been his into the room beyond, not hidden but never entered.

Tricked by his own mind into forgetting.

And there…there…

Terror of the past overlaid terror of the present. John's and Victor's experiences melded into one.

"Sherlock!"…help me!

But he couldn't focus on that now. He couldn't get lost in his mind. He forced himself out of the mind palace and back to the present. Later—later—he would deal with the fallout.

XXII.

When later finally arrived, after long, painful discussions with Mycroft and his parents, and Molly, and Lestrade, after reassuring and retrieving Mrs. Hudson and Rosie, after crashing at John's house once they remembered 221B's current state—Sherlock found that he wasn't very eager, after all, to return to the mind palace and explore the lost memories.

He'd always understood himself and Mycroft to have merely been different, at least partly by choice. No one made me; I made myself. And now, to learn that his whole life, his whole self, had been merely a response to childhood trauma…

Had he, somehow, suspected?

Do you ever wonder if there's something wrong with us? he'd asked Mycroft when faced with the world's grief and his own unsure response the death of someone he might, perhaps, care about.

All lives end, his brother had responded, All hearts are broken. Caring is not an advantage, Sherlock. And Mycroft, from his own perspective had not been wrong. Where had caring gotten him, except into an impossible situation between a mad, genius, psychopathic sister, and a damaged, genius, insecure brother? Yet caring had saved them all in the end. Had he not cared for her, Eurus would never have admitted John's location nor resubmitted to permanent incarceration.

He needed to remember. It was important. He closed his eyes and dove into the mind palace.

XXIII.

Sherlock was never the same again. But, perhaps, it was better that way.