Professor McGonagall turned to regard Harry. She didn't have a full adult Wrongdoing Face, but her expression was flat, controlled. "You must remember, Mr. Potter," she said, "that there was a war in this country not ten years ago. Everyone has lost someone, and to speak of friends dying in your arms - is not done lightly."
"I - I didn't mean to -" The inference dropped like a falling stone into Harry's exceptionally vivid imagination. He'd talked about someone breathing their last breath - and then the salesgirl had run away - and the war had ended ten years ago so that girl would have been at most eight or nine years old, when, when, "I'm sorry, I didn't mean to..." Harry choked up, and turned away to run from the older witch's gaze but there was a wall of dirt blocking his way and he didn't have his wand yet. "I'm sorry, I'm sorry, I'm sorry!"
There came a heavy sigh from behind him. "I know you are, Mr. Potter."
Harry dared to peek behind him. Professor McGonagall only seemed sad, now. "I'm sorry," Harry said again, feeling wretched. "Did anything like that happen to -" and then Harry shut his lips and slapped a hand over his mouth for good measure.
The older witch's face grew a little sadder. "You must learn to think before you speak, Mr. Potter, or else go through life without many friends. That has been the fate of many a Ravenclaw, and I hope it will not be yours."
Harry wanted to just run away. He wanted to pull out a wand and erase the whole thing from Professor McGonagall's memory, be back with her outside the shop again, make it didn't happen -
"But to answer your question, Mr. Potter, no, nothing like that has ever happened to me. Certainly I've watched a friend breathe their last, once or seven times. But not one of them ever cursed me as they died, and I never thought that they wouldn't forgive me. Why would you say such a thing, Mr. Potter? Why would you even think it?"
"I, I, I," Harry swallowed. "It's just that I always try to imagine the worst thing that could happen," and maybe he'd also been joking around a little but he would rather have bitten off his own tongue than say that now.
"What?" said Professor McGonagall. "But why?"
"So I can stop it from happening!"
"Mr. Potter..." the older witch's voice trailed off. Then she sighed, and knelt down beside him. "Mr. Potter," she said, gently now, "it's not your responsibility to take care of the students at Hogwarts. It's mine. I won't let anything bad happen to you or anyone else. Hogwarts is the safest place for magical children in all the wizarding world, and Madam Pomfrey has a full healer's office. You won't need a healer's kit at all, let alone a five-Galleon one."
"But I do!" Harry burst out. "Nowhere is perfectly safe! And what if my parents have a heart attack or get in an accident when I go home for Christmas - Madam Pomfrey won't be there, I'll need a healer's kit of my own -"
"What in Merlin's name..." Professor McGonagall said. She stood up, and looked down at Harry an expression torn between annoyance and concern. "There's no need to think about such terrible things, Mr. Potter!"
Harry's expression twisted up into bitterness, hearing that. "Yes there is! If you don't think, you don't just get hurt yourself, you end up hurting other people!"
Professor McGonagall opened her mouth, then closed it. The witch rubbed the bridge of her nose, looking thoughtful. "Mr. Potter... if I were to offer to listen to you for a while... is there anything you'd like to talk to me about?"
"About what?"
"About why you're convinced you must always be on your guard against terrible things happening to you."
Harry stared at her in puzzlement. That was a self-evident axiom. "Well..." Harry said slowly. He tried to organise his thoughts. How could he explain himself to a Professor-witch, when she didn't even know the basics? "Muggle researchers have found that people are always very optimistic, compared to reality. Like they say something will take two days and it takes ten days, or they say it'll take two months and it takes over thirty-five years. For example, in one experiment, they asked students for times by which they were 50% sure, 75% sure, and 99% sure they'd complete their homework, and only 13%, 19%, and 45% of the students finished by those times. And they found that the reason was that when they asked one group for their best-case estimates if everything went as well as possible, and another group for their average-case estimates if everything went as usual, they got back answers that were statistically indistinguishable. See, if you ask someone what they expect in the normal case, they visualise what looks like the line of maximum probability at each step along the way - everything going according to plan, with no surprises. But actually, since more than half the students didn't finish by the time they were 99% sure they'd be done, reality usually delivers results a little worse than the 'worst-case scenario'. It's called the planning fallacy, and the best way to fix it is to ask how long things took the last time you tried them. That's called using the outside view instead of the inside view. But when you're doing something new and can't do that, you just have to be really, really, really pessimistic. Like, so pessimistic that reality actually comes out better than you expected around as often and as much as it comes out worse. It's actually really hard to be so pessimistic that you stand a decent chance of undershooting real life. Like I make this big effort to be gloomy and I imagine one of my classmates getting bitten, but what actually happens is that the surviving Death Eaters attack the whole school to get at me. But on a happier note -"
"Stop," said Professor McGonagall.
Harry stopped. He had just been about to point out that at least they knew the Dark Lord wouldn't attack, since he was dead.
"I think I might not have made myself clear," the witch said, her precise Scottish voice sounding even more careful. "Did anything happen to you personally that frightened you, Mr. Potter?"
"What happened to me personally is only anecdotal evidence," Harry explained. "It doesn't carry the same weight as a replicated, peer-reviewed journal article about a controlled study with random assignment, many subjects, large effect sizes and strong statistical significance."
Professor McGonagall pinched the bridge of her nose, inhaled, and exhaled. "I would still like to hear about it," she said.
"Um..." Harry said. He took a deep breath. "There'd been some muggings in our neighborhood, and my mother asked me to return a pan she'd borrowed to a neighbor two streets away, and I said I didn't want to because I might get mugged, and she said, 'Harry, don't say things like that!' Like thinking about it would make it happen, so if I didn't talk about it, I would be safe. I tried to explain why I wasn't reassured, and she made me carry over the pan anyway. I was too young to know how statistically unlikely it was for a mugger to target me, but I was old enough to know that not-thinking about something doesn't stop it from happening, so I was really scared."
"Nothing else?" Professor McGonagall said after a pause, when it became clear that Harry was done. "There isn't anything else that happened to you?"
"I know it doesn't sound like much," Harry defended. "But it was just one of those critical life moments, you see? I mean, I knew that not thinking about something doesn't stop it from happening, I knew that, but I could see that Mum really thought that way." Harry stopped, struggling with the anger that was starting to rise up again when he thought about it. "She wouldn't listen. I tried to tell her, I begged her not to send me out, and she laughed it off. Everything I said, she treated like some sort of big joke..." Harry forced the black rage back down again. "That's when I realised that everyone who was supposed to protect me was actually crazy, and that they wouldn't listen to me no matter how much I begged them, and that I couldn't ever rely on them to get anything right." Sometimes good intentions weren't enough, sometimes you had to be sane...