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In Harry Potter and the Methods of Rationality by Eliezer Yudkowsky a concept called the 'Interdict of Merlin' appears: (all emphasis added)

Chapter 23:

His hand on the doorknob, Harry Potter already inside and waiting, wearing his cowled cloak.

"The ancient first-year spells," Harry Potter said. "What did you find?"

"They're no more powerful than the spells we use now."

Harry Potter's fist struck a desk, hard. "Damn it. All right. My own experiment was a failure, Draco. There's something called the Interdict of Merlin -"

Draco hit himself on the forehead, realizing.

"- which stops anyone from getting knowledge of powerful spells out of books, even if you find and read a powerful wizard's notes they won't make sense to you, it has to go from one living mind to another. I couldn't find any powerful spells that we had the instructions for but couldn't cast. But if you can't get them out of old books, why would anyone bother passing them on by word of mouth after they stopped working? Did you get the data on the Squib couples?"

Chapter 25:

Harry had asked Hermione about that earlier - on the train to Hogwarts, after hearing Draco say it - and so far as she knew, nothing more was known than the word itself.

It might have been pure legend. But it was also plausible enough that a civilization of magic-users, especially one from before the Interdict of Merlin, would have managed to blow itself up.

The line of reasoning continued: Atlantis had been an isolated civilization that had somehow brought into being the Source of Magic, and told it to serve only people with the Atlantean genetic marker, the blood of Atlantis.

and Chapter 80:

This is the Hall of the Wizengamot; there are older places, but they are hidden. Legend holds that the walls of dark stone were conjured, created, willed into existence by Merlin, when he gathered the most powerful wizards left in the world and awed them into accepting him as their chief. And when (the legend continues) the Seers continued to foretell that not enough had yet been done to prevent the end of the world and its magic, then (the story goes) Merlin sacrificed his life, and his wizardry, and his time, to lay in force the Interdict of Merlin. It was not an act without cost, for a place like this one could not be raised again by any power still known to wizardkind. Nor yet destroyed, for those walls of dark stone would pass unharmed, and perhaps unwarmed, through the heart of a nuclear explosion. It is a pity that nobody knows how to make them anymore.

Is there any mention at all of the Interdict of Merlin in Harry Potter canon (anything written, spoken of or referred to by J. K. Rowling), or was this simply a plot device added by Eliezer Yudkowsky?

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The Interdict is an added plot device (though something like it must surely exist elsewhere in the vast reaches of already-written fantasy).

I don't regard my own answers as canon when they haven't been recorded in the text itself, but Opinion of God is that the Interdict of Merlin applies to magical secrets that can directly or indirectly lead to wide-scale catastrophes if revealed. (It's possible that Harry's Brown Note for the Patronus Charm would fall into this category, even though it's not a secret that causes a nuclear-scale explosion as such.) Harry Potter and the Methods of Rationality includes instances of people learning relatively strong magic from books (e.g., Tom Riddle and the original horcrux spell), not to mention that Hermione is explicitly shown in-scene to have learned sixteen spells just from reading books. It's implied however that the art necessary to create e.g. the Deathly Hallows, or to raise Hogwarts, was unrecordable and therefore lost.

One way or another, some further assumption is necessary to explain why a literate society doesn't have more and more advanced Invisibility Cloaks every year, far trumping one that's a thousand years old, the way that broomsticks are shown to improve every year. In Harry Potter and the Methods of Rationality, this further assumption is the Interdict of Merlin. This allows a new faster broomstick to come out every year as in canon, while also allowing there to be ancient lost artifacts of unmatched power as in canon. The Interdict wasn't meant as a big weird plot point from nowhere, but as a lawful explanation of observations we'd already seen (one of the central tropes of rationalfic), which then had further consequences (also one of the central tropes of rationalfic).

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    whaaat... you made an account just to answer this question? I've always envisioned the Interdict as applying to powerful old (often dark) magic that is in the league more of people like Dumbledore or Voldemort, especially after reading a fanfiction of HPMOR which was based around finding a way to circumvent the Interdict. – user32390 Nov 26 '15 at 21:09
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    @MathiasFoster SPOILER: I think it refers to the act of making non-humanists unable to cast Patronusv1 if they know about the secret of Patronusv2. – March Ho Nov 27 '15 at 2:57
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    On an unrelated note, +1 for WoG answer on the fanfic side. – March Ho Nov 27 '15 at 2:58
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    You may want to merge this unregistered account if it is indeed yours. – user42419 Nov 27 '15 at 11:04
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    If you know what a Dementor is, you can't cast the animal Patronus. There are other versions of the spell that still work, but their conditions for casting are hard and neither Godric Gryffindor nor Rowena Ravenclaw figured them out, so they didn't tell anyone, so as not to render many people defenseless. The Interdict of Merlin might notice that danger, either directly, or because powerful wizards thought about it as a hazardous secret capable of causing widespread damage (I lean toward the latter hypothesis), and mark it as a secret that can't be transmitted though nonliving records. – Eliezer Yudkowsky Nov 29 '15 at 2:23
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Although I've not read Methods of Rationality, I can say with confidence that nothing like what is described appears in Rowling's original writings.

I hesitate to post an answer without offering supporting evidence, but it's hard to prove a negative; the best I can do is:

  • I've read the books many times, and have searched Pottermore, Accio-quote (which maintains transcripts of most Rowling interviews), and the Harry Potter Lexicon, to no avail

  • At least one (admittedly highly critical) review of MOR says so (expletive removed by me):

    For gibberflipping [Belgium]'s sake, Yudkowsky fabricated an entire core plot point--the Interdict of Merlin--because it suited him

The closest thing I've been able to find is an observation in a fan essay titled "The Limits of Magic", observing that knowledge cannot be transferred magically:

Skills and knowledge cannot be obtained by magical methods. There is no spell that professors can perform upon their students to fill their noggins with the requisite magical knowledge: still less can students perform such spells upon themselves when finals are approaching (or George and Fred would have surely already done so).

"The Limits of Magic" by Caius Marcius. Published on hp-lexicon.org

Even this isn't a statement made by Rowling, but an observation that, presumably, if such a thing were possible we would have seen someone do it.

  • Your point about Hermione at the beginning of the book is incorrect. The Interdict doesn't apply to low-level magic like she been demonstrating at that time: "There's something called the Interdict of Merlin which stops anyone from getting knowledge of powerful spells out of books [...]" – Jeremy Banks Nov 26 '15 at 20:16
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    @JeremyBanks How powerful is powerful enough? – Jason Baker Nov 26 '15 at 20:16
  • I can't remember how difficult the Protean charm is supposed to be, and the threshold is vague. It might be too powerful. But any early-year Hogwarts spells (like she's probably referring to in the first quote) would be below the bar. – Jeremy Banks Nov 26 '15 at 20:20
  • @JeremyBanks The Protean is described as "NEWT standard," so presumably your average Hogwarts graduate should be able to cast it. Regardless, I'm legitimately curious; is there a well-defined cut-off for how powerful a spell needs to be before it falls foul of the Interdict? – Jason Baker Nov 26 '15 at 20:23
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    It's almost embarrassing to have an answer that begins "Although I've not read Methods of Rationality" next to an answer from the actual author! Sez me, of course ;-) – Rand al'Thor Nov 30 '15 at 0:59
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Off the top of my head, I can think of a few examples of learning advanced magic from a written text.

The first would be everything Harry learned from the HBP Potion book, for example Sectumsempra seems to be quite a powerful spell.

The second is probably the most famous, the Horcruxes and how they are created. Other than gaining information about how the soul is split, Tom Riddle must have read all the information about Horcruxes from textbooks. We know he at least learned about them initially through a textbook.

The third is (in the Harry Potter universe) regarded to be one of the toughest spells to master: the Animagus form, learned by not one but three children almost definitely from a written text. (Who would they ask?)

As for your concerns about about ancient artifacts and why they couldn't recreate them, there are structures even now that we have no idea of how ancient civilisations built. Knowledge is often lost in the sands of time. I see no reason why even if the Peverell brothers recorded how they had created their objects that this recording of knowledge could not be destroyed or lost.

  • Yes but Magic does not seem to evolve in the same way Technology does, you can create new spells but if you don't right them down or show them off someone would have to recreate or rediscover that spell. Think of Dumbledores most famous discovery, the 12 uses of Dragon Blood. Also we have the Philosophers Stone the creative process for this was never recorded so that no one else could recreate it. Think of spells like lost languages, there are a fair few that even with modern computers we can't decipher. – CandiedMango Nov 26 '15 at 23:18
  • @MathiasFoster we don't know how to recreate Damascus steel. – OrangeDog Nov 27 '15 at 9:37
  • @orangedog or Greek fire – CandiedMango Nov 27 '15 at 9:40
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    But in both cases we can make much better steel and incendiary devices than those originals. Our average steel now is better than Damascus steel of that age was. Damascus steel wasn't so much a wonder in comparison to steels of this age, it was a wonder in comparison to the iron and crude steel weapons of the time (very brittle or very ductile). – Jim2B Nov 30 '15 at 6:33
  • It took several hundred years to create Napalm which is usually considered to be Greek Fire's modern day equivalent. Even so the recipe to Greek Fire is unknown. Also Demascus Steel has never been replicated it has been imitated but not replicated. The only reason we are able to replicate so much is due to our technological level of study and breaking down the elements as well as things that follow laws. We have no idea of the laws of HP magic there are but a few actually cited in canon. This is where HPMoR differs to HP from the little i read I know Harry is trying to set laws for Magic. – CandiedMango Nov 30 '15 at 6:51

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