What is the rules basis for the idea that characters don't know about mechanics like hit points, hit dice, and class levels?

Question

It's commonly said that values you find on a character sheet like hit points, hit dice, and class levels aren't diegetic. That is, the characters in the world that we're roleplaying in have no idea that hit points exist, that they have a pool of hit dice that can be spent, or that there is a moment to look forward to at which the cleric will gain a class level and get access to new spell levels.

However, I've never seen a proper rules basis for this grand assumption. I've seen people point to books, movies, and video games set in the worlds of the game like Faerûn, in which characters don't express knowledge about those facts. But this argument is flawed in two ways: first, those pieces of media are not game rules, and thus don't affect what our game characters can/do know; second, those pieces of media are cinematic and skip over plenty of things that must happen or be known in the world—you never see Indiana Jones go buy ammo or Luke Skywalker take a shower, but they must.

People will point to certain things that imply these things aren't known, but it's just as easy to point to other things in the rules that imply they are known. For example, a party who is out of hit dice and thus skips a short rest (or decides to take a long rest instead) is a group of characters making this decision based on something perceptible to themselves. Thus, an implication of a general rule by the existence of specific examples isn't sufficient; they could easily be exceptions to an implied opposite general rule, as many specifics in D&D are.

Is there anything in the rules, either for players or for dungeon masters, that explicitly states (not implies) that characters have no knowledge about these fundamental attributes of the way their world works?

Answer 1

In the Dungeon Master’s Guide.

The DMG contains a section on “Metagame Thinking” (p. 235):

Metagame Thinking

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Metagame thinking means thinking about the game as a game. It’s like when a character in a movie knows it’s a movie and acts accordingly. For example, a player might say, “The DM wouldn’t throw such a powerful monster at us!” or you might hear, “The read-aloud text spent a lot of time describing that door — let’s search it again!”

Discourage metagame thinking by giving players a gentle reminder: “What do your characters think?” You can curb metagame thinking by setting up situations that will be difficult for the characters and that might require negotiation or retreat to survive.

“Thinking about the game as a game” gets at what you’re describing in the question. That is, when the characters think about the rules of the game in the terms the rules use, this is metagame thinking, and the DMG directs DMs to discourage this sort of thinking.

We can further infer this idea from the basic understanding of the DM’s role (DMG, p. 4):

Every DM is the creator of his or her own campaign world. Whether you invent a world, adapt a world from a favorite movie or novel, or use a published setting for the D&D game, you make that world your own over the course of a campaign.

The world where you set your campaign is one of countless worlds that make up the D&D multiverse, a vast array of planes and worlds where adventures happen. Even if you’re using an established world such as the Forgotten Realms, your campaign takes place in a sort of mirror universe of the official setting where Forgotten Realms novels, game products, and digital games are assumed to take place. The world is yours to change as you see fit and yours to modify as you explore the consequences of the players’ actions.

We don’t need rules about metagame thinking here, we already have a DM who decides what the universe is like in the narrative. Characters don’t know about game mechanics because the DM has created a narrative in a world where those game mechanics don’t exist.

Answer 2

The DMG seems to suggest that it's not really supposed to be communicated directly, which in turn suggests that it's assumed that this information isn't really known in the world by the characters.

That is, if it should never be assumed you can figure out a monter's hit points, what're the odds that a character knows their own hit points and can communicate about it?

TRACKING MONSTER HIT POINTS

During a combat encounter, you need to track how much damage each monster takes. Most DMs track damage in secret so that their players don't know how many hit points a monster has remaining. Whether you choose to be secretive or not is up to you

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Players often ask how hurt a monster looks. Don't ever feel as though you need to reveal exact hit points, but if a monster is below half its hit point maximum, it's fair to say that it has visible wounds and appears beaten down.

Likewise, the Player's Handbook leaves describing what damage looks like up to the DM:

DESCRIBING THE EFFECTS OF DAMAGE

Dungeon Masters describe hit point loss in different ways. When your current hit point total is half or more of your hit point maximum, you typically show no signs of injury. When you drop below half your hit point maximum, you show signs of wear, sueh as euts and bruises. An attack that reduecs you to O hit points strikes you directly, leaving a bleeding injury or other trauma, or it simply knocks you unconscious.

Neither of these options even suggests "Or everyone just knows their hit points and those of others and talks about them", which isn't a direct rule against your idea of it being a known quantity in the universe, but I certainly would have expected them to show up here (or at least be acknowledged) if the designers considered such an idea at all plausible.

To me it means that the idea that you could do it didn't even cross anyone's mind, which is why it's never written down anywhere that you shouldn't.

Answer 3

There is no explicit statement about this

I first wanted to answer, as Thomas has pointed out, with the passage in the DMG about metagame thinking. However, on a close read, that passage talks about the players thinking about the game as a game, not about what the characters would know or would not know about things like hit dice. Both of the examples are about the narrative of the story - it would have been easy to make one about character observing game mechanics instead. I think it is the closest thing we have, but it is not explicit.

Even the fundamental loop of How To Play (PHB, p. 6), which explains the relationship of

1. The DM describes the environment
2. The players describe what they want to do.
3. The DM narrates the results of the adventurers’ actions.

does not explicitly address this (and it even muddles the player and the character -- strictly speaking, it should say "2. The players describe what they want their characters to do"). I think this is because this is such a natural assumption that the authors did not think to express it, and it is not normally a major issue for enjoyable play that needs resolution, although it can come up from time to time.

The best indication we have that there is this separation is that except in very rare examples, like the one TheLittlePeace cited, the language in the PHB rules always addresses the player or reader with "you" when it comes to game mechanics¹, it is not talking about the character. For example on page 6, PHB (italics for emphasis added):

Part 2 details the rules of how to play the game, beyond the basics described in this introduction.That part covers the kinds of die rolls you make to determine success or failure at the tasks your character attempts,

You can see the separation implied here: the player makes the rolls for the things the character attempts. The character is trying to do stuff, in the game world, the player is resolving them using the game rules. But it is really never 100% clear cut, as it is easy to mix up character and player in writing.

Counterexamples in the rules that suggest the characters know

There are also a few instances, where the rules require the characters to know about game mechanics to make decisions on using their abilities, such as the silvery barbs spell, which says:

Casting Time: 1 reaction, which you take when a creature you can see within 60 feet of yourself succeeds on an attack roll, an ability check, or a saving throw

Saving throws need not have an observable result, for example they do not if you try to charm someone. That means, the character to cast this spell must know what mechanics just happened. Now, that is awkward, and I think this spell, and a few other instances like it, are more of the exception that proves the rule, even if the rule is not spelled out explicitly.

Or, the Fighter's Know Your Enemy feature says:

Starting at 7th level, if you spend at least 1 minute observing or interacting with another creature outside combat, you can learn certain information about its capabilities compared to your own. The DM tells you if the creature is your equal, superior, or inferior in regard to two of the following characteristics of your choice:

• Strength score
• Dexterity score
• Constitution score
• Armor Class
• Current hit points
• Total class levels (if any)
• Fighter class levels (if any)

How does that work, if the fighter as a player character has no concept of what for example class levels are? It is not impossible to get around it – you can couch this in describing that he realizes the other fighter is "more experienced" than himself, but it takes some effort. Hit points are tricky, too, as they "represent a combination of physical and mental durability, the will to live, and luck" (PHB, p. 196), so what does the fighter learn here? That the other guy currently still has more will to live than he himself?

Lastly, there are section in the rules that make it sound like not the player is making the check, but the character, for example the Forage activity while traveling says

Forage. The character can keep an eye out for ready sources of food and water, making a Wisdom (Survival) check when the DM calls for it.

This reads natural, but really, it should be the character keeping an eye out, and the player making a check for the character, when the DM calls for it. But writing it like that is just too cumbersome. You don't want the extra verbiage every time the character tries to do something and a check has to be made.

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¹ Not always, though. The Combat chapter tells us that "you" in it can refer to either player or character depending on context:

Throughout this chapter, the rules address you, the player or Dungeon Master. The Dungeon Master controls all the monsters and nonplayer characters involved in combat, and each other player controls an adventurer. “You” can also mean the character or monster that you control.

Answer 4

The Player's Handbook's wording seems to imply that the characters know some meta knowledge

While Thomas's answer is correct, the PHB seems to have wording that implies that characters do in fact have some level of meta knowledge. (This might seem nit-picky, but wording here is important in my opinion)

First, it is important to note that there is a well known difference between player and character - the player being the human at the table, and the character being the one in the universe - detailed more or less on page 11 of the PHB: "Step-by-Step Characters". The entire chapter is about a player creating a character, and the wording is very specific.

So as a single example then, on page 186 of the PHB, under "Short Rest" (emphasis mine):

A character can spend one or more Hit Dice at the end of a short rest, up to the character's maximum number of Hit Dice, which is equal to the character's level. For each Hit Die spent this way, the player rolls the die and adds the character's Constitution modifier to it.

This implies that it is the character deciding to spend hit dice, which would mean that the character must be aware of it.

Answer 5

This concept is a fiction made up by people who want to play the game this way.

I commonly see people state that characters wouldn't know exact mechanisms of their abilities, attribute scores, hit points or hit dice, etc. This makes exactly zero sense to me. Beings in this fantasy world would create a taxonomy of these things in a way that described them.

Characters in my worlds are much closer to Milo in "Harry Potter and the Natural 20" than to not knowing anything about how things really work. In my world, Paladins know they can Lay Hands for healing of specific amounts, and know how many they have used in a day. Dwarves who took Dwarven Fortitude first of all decided to do so, and then know how many times they can use it per day to dodge and heal.

Neither method is right, and neither is supported solely by the rules. We all choose how we like to play the game. I'm an engineer and a world that can't be explained with math makes no sense to me. An artist might prefer a narrative world. Neither is right.