"Foolishness," Severus said softly. "Utter foolishness. The Dark Mark has not faded, nor has its master."
"See, that's what I mean by formally insufficient Bayesian evidence. Sure, it sounds all grim and foreboding and stuff, but is it that unlikely for a magical mark to stay around after the maker dies? Suppose the mark is certain to continue while the Dark Lord's sentience lives on, but a priori we'd only have guessed a twenty percent chance of the Dark Mark continuing to exist after the Dark Lord dies. Then the observation, 'The Dark Mark has not faded' is five times as likely to occur in worlds where the Dark Lord is alive as in worlds where the Dark Lord is dead. Is that really commensurate with the prior improbability of immortality? Let's say the prior odds were a hundred-to-one against the Dark Lord surviving. If a hypothesis is a hundred times as likely to be false versus true, and then you see evidence five times more likely if the hypothesis is true versus false, you should update to believing the hypothesis is twenty times as likely to be false as true. Odds of a hundred to one, times a likelihood ratio of one to five, equals odds of twenty to one that the Dark Lord is dead -"
"Where are you getting all these numbers, Potter?"
"That is the admitted weakness of the method," Harry said readily. "But what I'm qualitatively getting at is why the observation, 'The Dark Mark has not faded', is not adequate support for the hypothesis, 'The Dark Lord is immortal.' The evidence isn't as extraordinary as the claim." Harry paused. "Not to mention that even if the Dark Lord is alive, he doesn't have to be the one who framed Hermione. As a cunning man once said, there could be more than one plotter and more than one plan."
"Such as the Defense Professor," Severus said with a thin smile. "I suppose I must agree that he is a suspect. It was the Defense Professor last year, after all; and the year before that, and the year before that."
Harry's eyes dropped back to the parchment in his lap. "Let's move on. Are we certain that this Prophecy is accurate? Nobody messed with Professor McGonagall's memory, maybe edited or subtracted a line?"
Albus paused, then spoke slowly. "There is a great spell laid over Britain, recording every prophecy said within our borders. Far beneath the Most Ancient Hall of the Wizengamot, in the Department of Mysteries, they are recorded."
"The Hall of Prophecy," Minerva whispered. She'd read about that place, said to be a great room of shelves filled with glowing orbs, one after another appearing over the years. Merlin himself had wrought it, it was said; the greatest wizard's final slap to the face of Fate. Not all prophecies conduced to the good; and Merlin had wished for at least those spoken of in prophecy, to know what had been spoken of them. That was the respect Merlin had given to their free will, that Destiny might not control them from the outside, unwitting. Those mentioned within a prophecy would have an glowing orb float to their hand, and then hear the prophet's true voice speaking. Others who tried to touch an orb, it was said, would be driven mad - or possibly just have their heads explode, the legends were unclear on this point. Whatever Merlin's original intention, the Unspeakables hadn't let anyone enter in centuries, so far as she'd heard. Works of the Ancient Wizards had stated that later Unspeakables had discovered that tipping off the subjects of prophecies could interfere with seers releasing whatever temporal pressures they released; and so the heirs of Merlin had sealed his Hall. It did occur to Minerva to wonder (now that she'd spent a few months around Mr. Potter) how anyone could possibly know that; but she also knew better than to ask Albus, in case Albus tried to tell her. Minerva firmly believed that you only ought to worry about Time if you were a clock.
"The Hall of Prophecy," Albus confirmed lowly. "Those who are spoken of in a prophecy, may listen to that prophecy there. Do you see the implication, Harry?"
Harry frowned. "Well, I could listen to it, or the Dark Lord... oh, my parents. Those who had thrice defied him. They were also mentioned in the prophecy, so they could hear the recording?"
"If James and Lily heard anything different from what Minerva reported," Albus said evenly, "they did not say so to me."
"You took James and Lily there?" Minerva said.
"Fawkes can go to many places," Albus said. "Do not mention the fact."
Harry was staring directly at Albus. "Can I go to this Department of Mysteries place and hear the recorded prophecy? The original tone of voice might be helpful, from what I've heard."
Light glinted from the reflection of Albus's half-moon glasses as the old wizard slowly shook his head. "I think that would be unwise," Albus said. "For reasons beyond the obvious. It is dangerous, that place which Merlin made; more dangerous to some people than others."
"I see," Harry said tonelessly, and looked back down at the parchment. "I'll take the prophecy as assumed accurate for now. The next part says that the Dark Lord has marked me as his equal. Any ideas on what that means exactly?"
"Surely not," said Albus, "that you must imitate his ways, in any wise."
"I'm not dumb, Headmaster. Muggles have worked out a thing or two about temporal paradoxes, even if it's all theoretical to them. I won't throw away my ethics just because a signal from the future claims it's going to happen, because then that becomes the only reason why it happened in the first place. Still, what does it mean?"
"I do not know," said Severus.
"Nor I," she said.
Harry took out his wand, turned it over in his hands, gazing meditatively at the wood. "Eleven inches, holly, with a core of phoenix feather," Harry said. "And the phoenix whose tail feather is in this wand, only ever gave one other, which Mr... what was his name, Olive-something... made into the core of the Dark Lord's wand. And I'm a Parselmouth. It seemed like a lot of coincidence even then. And now I find out there's a prophecy stating that I'll be the Dark Lord's equal."
Severus's eyes were thoughtful; the Headmaster's gaze, unreadable.
"Could it be," Minerva said falteringly, "that You-Know-Who - that Voldemort - transferred some of his own powers to Mr. Potter, the night he gave him that scar? Not something he intended to do, surely. Still... I don't see how Mr. Potter could be his equal, if he had any less magic than the Dark Lord himself..."
"Meh," said Harry, still looking meditatively at his wand. "I'd fight the Dark Lord without any magic at all, if I had to. Homo sapiens didn't become the dominant species on this planet by having the sharpest claws or hardest armor - though I suppose some of that point may be lost on wizards. Still, it's beneath my dignity as a human being to be scared of anything that isn't smarter than I am; and from what I've heard, on that particular dimension the Dark Lord wasn't very scary."
The Potions Master spoke, his voice taking on some of his customary contemptuous drawl. "You imagine yourself more intelligent than the Dark Lord, Potter?"
"Yes, in fact," said Harry, pulling back the left sleeve of his robes, and rolling up the shirtsleeve beneath to expose the bare elbow. "Oh, that reminds me! Let's make sure nobody here has the clearly visible tattoo in the standard, easily checkable location which would mark them as a secret enemy spy."
Albus made a quieting gesture that halted the Potions Master before he could say anything scathing. "Tell me, Harry," Albus said, "how would you have crafted the Dark Mark?"
"Nonstandard locations," Harry said promptly, "not easily found without embarrassment and fuss, though of course any security-conscious person would check anyway. Make it smaller, if possible. Overlay another non-magical tattoo to obscure the exact shape - better yet, cover it with a layer of fake skin -"
"Cunning indeed," Albus said. "But tell me, suppose you could craft any conditions you wished into the Mark, fading it or raising it as you wished. What would you do then?"
"Make it completely invisible at all times," Harry said in tones of stating the obvious. "You don't want there to be any detectable difference between a spy and a non-spy."
"Suppose you are more cunning still," Albus said. "You are a master of trickery, a master of deception, and you employ your abilities to the fullest."
&nb
sp; "Well -" The boy stopped, frowning. "It seems unnecessarily complicated, more like a tactic a villain would use in a role-playing game than something you'd try in a real-life war. But I suppose you could put fake Dark Marks on people who aren't really Death Eaters, and keep the Dark Marks on the real Death Eaters invisible. But then there's the question of why people would start believing in the first place that the Dark Mark identified a Death Eater... I'd have to think about it for at least five minutes, if I were going to take the problem seriously."
"I ask you this," Albus said, still in that mild tone, "because I did indeed, in the early days of the war, perform such tests as you suggested. The Order survived my folly only because Alastor did not trust in the bare arms we saw. I had thought, afterward, that the bearers of the Mark might hide it or show it at their will. And yet when we hied Igor Karkaroff before the Wizengamot, that Mark showed clear on his arm, for all that Karkaroff wished to protest his innocence. What true rule may govern the Dark Mark, I do not know. Even Severus is still bound by his Mark not to reveal its secrets to any who do not know them."
"Oh, well that makes it obvious," Harry said promptly. "Wait, hold on - you were a Death Eater?" Harry transferred his stare to Severus.
Severus returned a thin smile. "I still am, so far as they know."
"Harry," said Albus, eyes only for the boy. "What do you mean, that makes it obvious?"
"Information theory 101," the boy said in a lecturing tone. "Observing variable X conveys information about variable Y, if and only if the possible values of X have different probabilities given different states of Y. The instant you hear about anything whatsoever that varies between a spy and a nonspy, you should immediately think of exploiting it to distinguish spies from nonspies. Similarly, to distinguish reality from lies, you need a process which behaves differently in the presence of truth and falsehood - that's why 'faith' doesn't work as a discriminant, while 'make experimental predictions and test them' does. You say someone with the Dark Mark can't reveal its secrets to anyone who doesn't already know them. So to find out how the Dark Mark operates, write down every way you can imagine the Dark Mark might work, then watch Professor Snape try to tell each of those things to a confederate - maybe one who doesn't know what the experiment is about - I'll explain binary search later so that you can play Twenty Questions to narrow things down - and whatever he can't say out loud is true. His silence would be something that behaves differently in the presence of true statements about the Mark, versus false statements, you see."
Minerva's mouth was hanging open, she realized; and she closed it abruptly. Even Albus looked surprised.
"And after that, like I said, any behavioral difference between spies and nonspies can be used to identify spies. Once you've identified at least one magically censored secret of the Dark Mark, you can test someone for the Dark Mark by seeing if they can reveal that secret to somebody who doesn't already know it -"
"Thank you, Mr. Potter."
Everyone looked at Severus. The Potions Master was straightening, his teeth bared in a grimace of angry triumph. "Headmaster, I can now speak freely of the Mark. If we know we are caught for a Death Eater, before others who have not yet seen our bare arms, our Mark reveals itself whether we will it or no. But if they have already seen our arms bare, it does not reveal itself; nor if we are only being tested from suspicion. Thus the Dark Mark seems to identify Death Eaters - but only those already found, you perceive."
"Ah..." Albus said. "Thank you, Severus." He closed his eyes briefly. "That would indeed explain why Black escaped even Peter's notice... ah, well. And Harry's proposed test?"
The Potions Master shook his head. "The Dark Lord was no fool, despite Potter's delusions. The moment such a test is suspected, the Mark ceases to bind our tongues. Yet I could not hint at the possibility, but only wait for another to deduce it." Another thin smile. "I would award you a good many House points, Mr. Potter, if it would not compromise my cover. But as you can see, the Dark Lord was quite cunning." His gaze grew more distant. "Oh," Severus breathed, "he was very cunning indeed..."
Harry Potter sat still for a long moment.
Then -
"No," Harry said. The boy shook his head. "No, that can't actually be true. First of all, we're talking about the kind of logic puzzle that would appear in chapter one of a Raymond Smullyan book, nowhere near the level of what Muggle scientists do for a living. And second, for all I know, it took the Dark Lord five months of thinking to invent the puzzle I just solved in five seconds -"
"Is it that inconceivable to you, Potter, that anyone could be so intelligent as yourself?" The Potions Master's voice held more curiosity than scorn.
"It's called a base rate, Professor Snape. The evidence is equally compatible with the Dark Lord inventing that puzzle over the course of five months or over the course of five seconds, but in any given population there'll be many more people who can do it in five months than in five seconds..." Harry pasted a hand against his forehead. "Darn it, how can I explain this? I suppose, from your perspective, the Dark Lord came up with a clever puzzle and I cleverly solved it and that makes us look equal."
"I remember your first day of Potions class," the Potions Master said dryly. "I think you have a ways still to go."
"Peace, Severus," Albus said. "Harry has already accomplished more than you know. Yet tell me, Harry - why do you believe the Dark Lord is less than you? Surely he is a damaged soul in many ways. But cunning for cunning - you are not yet ready to face him, I would judge; and I know the full tally of your deeds."
The frustrating thing about this conversation was that Harry couldn't say his actual reasons for disagreeing, which violated several basic principles of cooperative discourse.
He couldn't explain how Bellatrix had really been removed from Azkaban - not by You-Know-Who in any guise, but by the combined wits of Harry and Professor Quirrell.
Harry didn't want to say in front of Professor McGonagall that the existence of brain damage implied that there were no such things as souls. Which made a successful immortality ritual... well, not impossible, Harry certainly intended to forge a road to magical immortality someday, but it would be a lot harder and require much more ingenuity than just binding an already-existent soul to a lich's phylactery. Which no intelligent wizard would bother doing in the first place, if they knew their souls were immortal.
And the true and honest reason Harry knew the Dark Lord couldn't have been that smart... well... there wasn't any tactful way to say it, but...
Harry had been to a convocation of the Wizengamot. He'd seen the laughable 'security precautions', if you could call them that, guarding the deepest levels of the Ministry of Magic. They didn't even have the Thief's Downfall which goblins used to wash away Polyjuice and Imperius Curses on people entering Gringotts. The obvious takeover route would be to Imperius the Minister of Magic and a few department heads, and owl a hand grenade to anyone too powerful to Imperius. Or owl them knockout gas, if you needed them alive and in a state of Living Death to take hairs for Polyjuice potions. Legilimency, False Memories, the Confundus Charm - it was ridiculous, the magical world was supersaturated with ways to cheat. Harry might not do any of those things himself, during his own takeover of Britain, since he was constrained by Ethics... well, Harry might do some of the lesser ones, since Polyjuice or a temporary Confundus or read-only Legilimency all sounded better than an extra day of Azkaban... but...
If Harry hadn't been constrained by Ethics, it was possible he could've wiped out the eviller sections of the Wizengamot that day; all by himself, using only a first-year's magical power, on account of being clever enough to figure out Dementors. Though Harry might not have been in such a great political position after that, the surviving Wizengamot members might've found it easy and cheap to disavow his actions for P.R. purposes and condemn him, even if the smarter ones realized it was for the greater good... but still.
If you were completely unrestrained by ethics, armed with the ancient secre
ts of Salazar Slytherin, had dozens of powerful followers including Lucius Malfoy, and it took you more than ten years to fail to overthrow the government of magical Britain, it meant you were stupid.
"How can I put this..." Harry said. "Look, Headmaster, you've got ethics, there's a lot of battle tactics you don't use because you're not evil. And you fought the Dark Lord, a tremendously powerful wizard who wasn't so restrained, and you held him off anyway. If You-Know-Who had been super-smart on top of that, you'd be dead. All of you. You'd have died instantly -"
"Harry," Professor McGonagall said. Her voice was faltering. "Harry, we almost did all die. More than half the Order of the Phoenix died. If not for Albus - Albus Dumbledore, the greatest wizard in two centuries, Harry - we surely would have perished."
Harry passed a hand across his forehead. "I'm sorry," Harry said. "I'm not trying to minimize what you went through. I know that You-Know-Who was a completely evil, incredibly powerful Dark Wizard with dozens of powerful followers, and that's... bad, yes, definitely bad. It's just..." All that isn't on remotely the same threat scale as the enemy being smart, in which case they Transfigure botulinum toxin and sneak a millionth of a gram into your teacup. Was there any safe way to convey that concept without citing specifics? Harry couldn't think of one.
"Please, Harry," said Professor McGonagall. "Please, Harry, I beg you - take the Dark Lord seriously! He is more dangerous than -" The senior witch seemed to be having trouble finding words. "He is far more dangerous than Transfiguration."
Harry's eyebrows went up before he could stop himself. A dark chuckle came from Severus Snape's direction.
Um, said the voice of Ravenclaw within him. Um, honestly Professor McGonagall is right, we're not taking this as seriously as we'd take a scientific problem. The difficult thing is to react at all to new information, instead of just flushing it out the window. Right now it looks like we didn't shift belief at all after encountering an unexpected, important argument. Our dismissal of Lord Voldemort as a serious threat was originally based on the Dark Mark being blatantly stupid. It would require a focused effort to de-update and suspect the whole garden-path of reasoning we went down based on that false assumption, and we're not putting in that effort right now.
Harry Potter and the Methods of Rationality Page 146