NOTES;
I DO NOT believe Sherlock has prosopagnosia in the BBC canon; I think there's enough evidence to prove otherwise. However, I wanted to entertain the notion because I do have it myself, and it does result in me noticing lots of random details about people. I am not anywhere near Sherlock's level, but I can dream.

Also, this is the first fanfiction I've written and published in several years, so comments are much appreciated.

"I've told you before. You see, but you don't observe."

John furrowed his brow, looking at Sherlock over his tuna sandwich. The detective had just wrapped up another successful case, much to Lestrade's relief, and now the pair was catching up on important things, such as lunch. Or rather, John was catching up on lunch, and Sherlock was engaging in his favorite pastime of deducing things about random people in the cafe.

John was familiar with Sherlock's methods, but that didn't mean he understood them, and in voicing his puzzlement, received Sherlock's obvious response.

"I know that. What I mean is... How come you're like that? You forget your own birthday, but you notice all these tiny details about people. Why?"

Sherlock leaned back in his seat, his face assuming a contemplative expression. For a moment, John thought his friend would lapse into the familiar silence he associated with falling into deep thought, but surprisingly, he spoke up.

"John… how do you know a person?" he asked, tapping his fingertips on the tabletop.

"What?"

"You look at them once, and you remember their face, correct?"

"Of course."

"I don't." Sherlock brought his hands together and clasped them in front of himself. "The moment I look away from someone, their face evaporates from my mind."

"So you delete that information? A face is as unimportant as the solar system, is it?" John replied, merely entertaining Sherlock's line of thought, but admittedly he was baffled. How was that possible? Moreover, he remembered John, right?

"No. As antisocial as I am, the ability to remember people is vital." There was something visibly nervous about Sherlock's demeanor as he spoke- not overtly so, but enough that John knew he was prying into something quite personal. He didn't know why Sherlock had chosen to open up now, but it seemed as if it had been on his mind for a long time and needed to be said. "There's a term for it," Sherlock continued after a pause. "Prosopagnosia. Did you never come across that in medical school?"

"Now that you mention it," said John cautiously, "maybe I have."

"Very well. Now imagine this. You're a kid, and all of your schoolmates wear the same uniform. They're all neat, posh little boys, they all have similar hairstyles. How do you tell them apart if not by their faces?" Sherlock smiled, but it betrayed sadness. "I didn't realize it when I was young. I had no reason to believe I was different. But there is a distinct rift that happens between children when you can't properly integrate into a group."

John nodded. "Is that why you had no friends?"

"I can't say that was the only reason." Sherlock returned to a neutral expression. "I never particularly wanted them, either. But it wasn't easy."

"So… the observation part?" John added, returning to the real topic of the conversation.

"It comes with the territory of never knowing exactly who you're seeing. You, for example. When we first met, you remember what I deduced about you? Army doctor, recently returned from Afghanistan. With that information, every time I saw you again, and confirmed the previous information I had gathered, I could be reasonably sure it was you."

John paused to consider this information.

"So… you really couldn't remember me?"

"Your face. I couldn't remember your face. But I knew a man like you by the name of 'John', and for all intents and purposes, that was good enough."

"I never imagined it was like that." John averted his eyes. The whole conversation had left an awkward atmosphere between them.

"After many years, it became natural. I don't see faces, but I observe the person. I don't even have to think anymore. I see a collection of facts. Their manner of walking, their posture. Hairstyle, clothes, skin, eyes. What they've been doing recently, their most likely profession, their social roles. There can be only one person who fits all the facts at once. And as you can see, this condition has left some pleasant side effects."

"Like allowing you to be a detective."

Sherlock tapped his fingers again, and assumed a distant and dreamlike gaze, which John understood to mean he had something very important to think about, and that unlike before he wasn't about to speak anytime soon.

John finished his lunch. He would never fully understand, but he knew enough, and he was satisfied.