It's frankly hard to say what's at the core of this discussion anymore. What I do know is that if you want to call "thinking" an action, then because of the rule that players determine what the characters think, then there can be no ability check here since there is no uncertainty as to the outcome. The character thinks what the player says he or she thinks.
*
sigh*
You know, I know why we keep going in this circle. Because you could care less about players using out-of-character knowledge, seemingly in any form. But this is also why a lot of people see Intelligence as a dump stat, because two of the biggest uses for Intelligence are Investigation and knowledge checks. Oh sorry, Intelligence checks using proficiency with the intent to recall lore. But, if players get to determine that they already know the lore, then there is no need for those checks.
If they just tell you they know something, then that is what they know. The only check upon that is that they might be wrong out of the game because you as the DM changed something. In which case, why do we even bother to have an Intelligence stat and the skills for recalling various types of lore. It seems meaningless under this style.
I don't see any complications with that rule. You don't ask for checks if there's no meaningful consequence for failure. Easy peasy.
Except what counts as a "meaningful consequence"? Not knowing something obviously isn't meaningful enough, taking a lot of time is probably not meaningful enough especially if players don't take these checks while under a time pressure or in dangerous territory. You have to actively work to make things worse for the players in response to them attempting things, just to allow them to make checks, or they auto-succeed on trying anything.
Sure, "the players just succeed" sounds really easy on paper, but it opens things to abuse that I don't want to deal with, and makes failing a roll dangerous enough that my players might not end up attempting interesting things. After all, who would try and woo a princess if failing the charisma check ends up with her ordering your execution. After all, her just not being interested isn't "meaningful" enough, you have to end up making things worse for you and your party.
The characters, as established by the players, think that earth elementals are vulnerable to thunder damage. There is no uncertainty here and thus no check. They might be right, they might be wrong, but there is no action declaration here to recall lore.
If you call for a check, you are de facto stating that the characters are attempting to perform a task with an uncertain outcome and meaningful consequence for failure because that is when the rules say the DM calls for a check. But only the players may describe what they want their characters to do. If the DM does it, that DM is overstepping his or her role.
The first paragraph is our point of disagreement. Players cannot just tell me they successfully recall lore. Recalling lore is an action, it has associated skill proficiencies. The fact that they might be right or they might be wrong tells us that there is uncertainty about that. Again, you as the DM are free to make changes, these earth elementals summoned by Zuul might be different, but the players have recalled facts about normal earth elementals because they have either fought them in other games or read the MM. That is not knowledge their characters are just born knowing.
So, if I call for a check, I am not telling the player what they are doing. They already declared the action, I am adjudicating. That is not overstepping my bounds.
The players in this example didn't describe their characters as performing a task to recall lore or make deductions, so there's no check in the first place and no need to determine if there are meaningful consequences for those tasks.
You are right, they didn't explicitly say "I try to remember what the vulnerabilities of earth elementals are." Instead, they just declared "I know that Earth Elementals are weak to Thunder damage."
So... if they player just tells you they succeed and get the end result, they don't need to make a check? That is ludicrous. You would never allow a player to simply state "I walk off with the Queen's Crown" and just let them do so, why then do we allow them to state "I perfectly recalled the weaknesses of this monster"?
They are attempting to do something with uncertainty, we at the table do not know if this character has this knowledge, so a check is called for.
"Meaningful consequences" are determined by the context of the situation in which the PCs find themselves, so in the abstract it's not easy to say what might be a meaningful consequence for failure on a task to recall lore or make deductions. In context, however, it might be very important to be able to recall a fact and that failing to do so has meaningful consequences. In many cases, however, there won't be and so the DM just says whether the character recalls the lore or makes the deduction. Maybe he or she does and maybe he or she doesn't.
So, not knowing the weakness of Earth Elementals and not being able to prepare for the coming fight by buying scrolls specifically targeting that weakness is not meaningful enough? How much more impactful does a consequence have to become to be meaningful?
In the example I gave upthread from my Eberron game, the meaningful consequence for failure of figuring out if the substance covering the boxes was indeed brown mold was damage. The warforged was using his integrated tool to investigate it. With a failure on the check, the experiment goes awry, the brown mold grows at an exponential rate, shatters the test tube, and the character takes some cold damage. However, that would have been ruled as progress combined with a setback. The substance is confirmed to be brown mold, but at the cost of some hit points. As it happens, the check succeeded.
As you do not like the rule for meaningful consequences for failure being a requirement of a check and you also appear to declare actions for the characters so far as I can tell, then yes, you will most likely have more Intelligence checks in your game than in mine. But that doesn't mean my game has none. Verifying one's assumptions often entails recalling lore or making deductions, after all.
Two things.
First, Holy crap. That was an integrated tool system, which means it was in the warforged's
warm body. A failure led to them getting infested with quickly growing brown mold within their body, the only way to destroy said mold being to expose it to cold damage, so the player is either going to constantly be draining hp from taking the cold damage of being "near" the mold that is inside their body, or have themselves blasted with cold damage to destroy the mold. Cold damage which would have been Ice Knife from the wizard if I remember your example correctly. An Ice Knife which potentially would have needed to be targeted within the warforged's body to hit the mold growing over their integrated tool.
Did the warforge know they were courting death, loss of their tool set, and possible dismemberment from exploding ice shards when they tried to determine the nature of a mold? A mold they strongly suspected the nature of?
Secondly, how are they supposed to verify their assumptions with checks to recall lore? They are only thinking, which means they automatically succeed, because there are no meaningful consequences. Unless they could be wrong about what they are thinking... which is kind of the entire point of me calling for a check involving Arcana in the first place. So since you disagree with me, there must be something else players do in your game to recall lore. They cannot just make a check, because they cannot fail to think something they want to think.
Perhaps under your table rules, that is the case. But under the rules of the game, the DM is not properly adjudicating the only action on the table - the barbarian going to buy the scrolls. The reason given for doing so is completely irrelevant to the adjudication process. If the player has said nothing about why the character wanted to buy the scrolls, would you have asked for a check?
As we established before, I would likely ask them why a character who cannot use magic scrolls is going to go and buy magic scrolls. This would likely get their intent, which brings us back to the beginning of this discussion.
I neither like nor dislike your usage of terms. I point out that it doesn't exist in this game, but does exist in other games, because a lot of DMs in my experience do not revise their approaches when moving from game to game to, in my view, the detriment of their understanding and discussions of the game they are playing. Asking for a "knowledge check" before the player even declares an action to recall lore or make a deduction is an example of this.
But it does exist in this game. Players can make checks using Intelligence (Arcana) to recall the weaknesses of monsters. This is a knowledge check. "A rose by any other name" as it were. Sure, the game doesn't explicitly call them that, but since players can recall lore it is assumed that they don't know every fact about monsters, so what is the difference here? Did 3.5 have a rule that explicitly said "Players are not expected to have all the information on a monster from the monsters statblock, if they wish to use this information, they should make a knowledge check" and 5e says somewhere that I've missed that "players are expected to know a monster's statblock and do not need a check to recall lore about monsters"?
I mean, you keep saying that the rules allow the player to know this stuff, but the rules never state that. At least, not that I've ever found. The very existence of Arcana, Religion, Nature, and History seem to contradict this opinion that players can just know whatever they wish to know.
I think there's some bias at play here. While I've seen this sort of thing go wrong, I've also seen it go right. It depends on the players. My regulars would certainly be fine with it because of the culture at our table in which the expectation is that you accept and build while pursuing the goals of play.
Accept and build what? That there can't be secrets? That players will act on knowledge there is no way their character's could know?
I admit, I have biases, but I don't see how this improves fun at the table, if players can just know everything that happens no matter where their character is or what is happening.
I worry about achieving the goals of play, that is, everyone having a good time and helping to create an exciting, memorable story as a result of play by performing the role of DM to the utmost of my ability.
Anything else seems superfluous, especially what some people call "metagaming." That is a self-inflicted problem in my view.
But you seem unconcerned with actions that can impact people having a good time and creating an internally consistent story.
In fact, your description of a DM is very hands-off in every aspect.
On one hand, one could say this appears to be a player who does not understand or buy into the genre (e.g. sword and sorcery) and that warrants an out-of-game discussion. On the other hand, the DM can simply adjudicate the action by having the shipwright explain that the sort of thing the PC wants to do isn't possible. The shipwright might believe that such a ship would never float and, besides, there isn't that much steel in all the kingdoms of the realm to build something that big. "Now stop wasting my time, you lunatic!"
What I wouldn't do is tell the player he or she can't do that or ask for a check to invalidate the action declaration.
So players can bring modern designs, knowledge of chemistry, gunpowder, ect to the game. You may talk to them out of game, maybe pausing the game to tell them that isn't consistent with the world... but that's exactly what happens by telling them "No, your character wouldn't know that" when it comes to these applications. Because pausing the game and talking to them is telling them that that knowledge is not acceptable in the game world because it doesn't match the knowledge that exists in the game world.
The other option, of letting them keep the knowledge and simply not have people believe them, leaves open the chance for them to try and make the thing themselves. Personally revolutionizing the economy is incredibly lucrative, and if you are trying to run a single game world for many different campaigns, it is incredibly destabilizing.