This is not a good argument from a legalistic point of view.
It's a good thing 5e isn't a 'legalistic' RPG ruleset, like Pathfinder, then.
I'm serious -- 5e has deliberately chosen to define very few general terms and provide a limited number of general rules. If you follow Jeremy Crawford on Twitter, you're no doubt aware of the number of times he says something along the lines of "there is no general rule for 'x' in Fifth Edition'.
The key insight is this:
Sage Advice Compendium said:
Many unexpected things can happen in a D&D campaign, and no set of rules could reasonably account for every contingency. If the rules tried to do so, the game would become unplayable. An alternative would be for the rules to severely limit what characters can do, which would be counter to the open-endedness of D&D. The direction we chose for the current edition was to lay a foundation of rules that a DM could build on, and we embraced the DM’s role as the bridge between the things the rules address and the things they don’t.
CapnZapp said:
I don't contest your conclusion, that the DM is authorized to make rulings. However, the DM is authorized to make rulings because the rules say so, not because the rules does not specify every circumstance.
I disagree -- as should be obvious from the quote above, the game is *designed* for the DM to make rulings where the rules do not (and cannot) cover all contingencies:
Sage Advice Compendium said:
The game’s rules are meant to help organize, and even inspire, the action of a D&D campaign. The rules are a tool, and we want our tools to be as effective as possible. No matter how good those tools might be, they need a group of players to bring them to life and a DM to guide their use.
CapnZapp said:
The fact that Invisibility does not specify every cause for it to end should not be taken as license to interpret it as "anything" can end it.
But, and this is the point, if a player presents a reasonable scenario under which Invisibility should end, and the DM agrees, then even though that scenario isn't presented in the rules, it is still allowed (and I would argue, expected) that the DM and player will agree that Invisibility would end. The rules do not stand in the way of the players and DM agreeing on their game experience.
Where the DM and player do not agree on their game experience, the player can appeal to the rules, but it is the responsibility of the DM to determine if the rules apply. As an example, you note that a hypothetical adventure might provide a different exception to ending Invisibility, which makes sense -- but the player would not necessarily know that the adventure provides an exception. Since only the DM is privy to all the rules in this case, it is the DM's job to adjudicate this disagreement.
But once you've accepted that the DM is allowed to make rulings when she has information that the players do not, it becomes a hazier question -- after all, the adventure, while approved for Adventurer's League, wasn't published by the D&D rules team, or even technically by WotC given the current use of DMs Guild as the AL adventure distrubution system. There are plenty of adventures that simply get the rules wrong -- such as the Season 5 adventure that calls for players to gain 'resistance 3' or 'resistance 5' rather than 'resistance' as defined in the core rules.
So there is frequently disagreement about the rules: disagreement between players and DMs over how to interpret spell descriptions, disagreement between adventures and the core rulebooks on how to adjudicate mechanics, even disagreement between posters on an internet discussion board on how the rules are organized and designed. To keep the game from devolving into a constant series of arguments over the rules, the AL explicitly bestows the power to decide 'what the rules are' to the DM. The DM isn't supposed to decide rulings based on whether or not she 'likes' the rules in the PH or adventure -- the AL does specify that DMs are expected to run the game according to the core rules -- but the crux of the problem is that nobody from the AL admin team is on hand to overrule the DM if she does decide to overrule a rule she doesn't like, and the players do not have the authority to overrule the DM. If you're looking for a 'legalistic' argument, that's the one to use.
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Pauper