Should Insight be able to determine if an NPC is lying?

D1Tremere said:

The player always determines what the character thinks, does, and says, EXCEPT when those things are determined by the rules or the DM.

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Short of magical compulsion which is an exception to the rule, the player always determines what the character thinks, does, and says. The DM can only describe the environment and narrate the result of the adventurers' actions.

D1Tremere said:

They are always bound by the results of checks (or do players in your game simply disbelieve damage and hits away?).

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That's not the same thing as the DM telling a player, for example, that he or she must have the character act as if the NPC is lying (or telling the truth).

D1Tremere said:

A DM can choose to disregard the outcome of the rules/roles, and a player can choose to roleplay a scenario without invoking roles in many cases, but ultimately a player roles dice to try and succeed at a task or suffer the outcomes of failure. Why else are the dice there?

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The dice are there to resolve uncertainty as to the outcome of a task proposed by the player when there's a meaningful consequence of failure. The DM determines whether or not there is uncertainty based on what the player has described as wanting to do. The player does not choose to make ability checks. That is solely the DM's call. If anything, it's not very smart play for a player to want to roll the dice as the d20 is very unpredictable. If success is the player's goal, then trying to avoid the dice is a better strategy.

D1Tremere said:

The skill actually says “Your Wisdom (Insight) check decides whether you can determine the true intentions of a creature.” Note that it says the check decides what your character determines (which is telling them what they think). What exactly would your character’s thoughts be based on in such a scenario if not what they observed in their environment?

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An ability check is not a task.

The ability check, to which the Insight skill proficiency may apply, resolves uncertainty as to the task the player proposed. The task is described if you continue reading the entry for Insight - "...gleaning clues from body language, speech habits, and changes in mannerisms." It has everything to do with observing another creature. The DM is telling you what you observe, just like when he or she is describing the environment, not what your character thinks.

D1Tremere said:

A DM can’t FORCE a player to make a roll. A player can choose to make a role in pursuit of a mechanical task, which is why the rules exist in the first place. You can choose for your character that they believe someone is lying or telling the truth just as anyone can eschew engaging with reality when conjuring their beliefs and opinions. If you want to pretend your character knows what is what that is fine, but once you engage with reality is has a nasty way of conflicting with such beliefs.

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A "roll" is not a task and players can't choose to make ability checks. That is the DM's call and he or she makes that call when the task proposed by the player for the character has an uncertain outcome and a meaningful consequence for failure.

At least, that's what the rules say. You can choose to play it some other way, of course.

D1Tremere said:

To clarify my position, a DM certainly CAN choose what actions succeed or fail, but that involves disregarding the rules. That is something they are free to do (and should do in many cases), but it is not the default of each action generally. To make success and failure an arbitrary extension of the DM would create a lot of problems, not the least of which would be invalidating every mechanical choice that players made for their characters. To disregard a check here or there or ignore a rule when it suits a scene is fine. To reduce every character to an avatar of the player with no mechanical strengths or weaknesses would be playing a game other than 5e.

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I'm sorry, but none of that is true, except perhaps at your table (and probably others). The rules come into play when the DM says they do since they serve the DM and not the other way around. The "default" is that the players say what they want to do and the DM narrates the results. Sometimes, when certain criteria are met in the eyes of the DM, the DM calls for a roll. A character's "mechanical strengths and weaknesses" come into play if and when the player proposes a task that has an uncertain outcome and a meaningful consequence for failure, which they will do quite frequently if the player is portraying a bold adventurer confronting deadly perils. But otherwise, success, failure, and uncertainty are all completely in the control of the DM.

Out of curiosity, did you play much D&D 3.Xe or D&D 4e?

iserith said:

I'm sorry, but none of that is true, except perhaps at your table (and probably others)

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You could be right, I have been known to make mistakes and I do run my games in a certain way. I would argue that the results of the poll suggest that my interpretation is more in line with the default at most peoples tables. All of that said, I will defend my position a bit with actual rulebook information.

"Ability Checks An ability check is a test to see whether a character succeeds at a task that he or she has decided to attempt." Says right here that the player can decide to attempt an ability check whenever they want to see if they succeed or fail at a task. Says nothing about the DM making that decision.

Your quote about Insight ignores the point. As the skill specifically states, the check decides (not the player, not the DM) what your character determines. The part you emphasize just details in game fluff that explains WHY/HOW the check determines. You also avoided my question. If the check is your observation of the environment, what else would determine your thoughts if not that?

I would also argue that the rules serve both the DM and the Players as "As the player who creates the game world and the adventures that take place within it, the DM is a natural fit to take on the referee role. As a referee, the DM acts as a mediator between the rules and the players."
Your reference to the rules serving the DM and not the other way around is used in the rulebooks to say that the DM doesn't serve the rules, an explanation that they need not be slavish to them. It does not refer to the players relationships to the rules. If the DM is a referee that would suggest that the rules exist between themselves and the players.

I'm tempted to add a snarky comment here to address yours, but I honestly enjoy discussing my perspective with others and see no need to make it less fun.

D1Tremere said:

You could be right, I have been known to make mistakes and I do run my games in a certain way. I would argue that the results of the poll suggest that my interpretation is more in line with the default at most peoples tables. All of that said, I will defend my position a bit with actual rulebook information.

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I voted "Yes" on the poll.

D1Tremere said:

"Ability Checks An ability check is a test to see whether a character succeeds at a task that he or she has decided to attempt." Says right here that the player can decide to attempt an ability check whenever they want to see if they succeed or fail at a task. Says nothing about the DM making that decision.

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This is a common error people make, especially if they played D&D 3.Xe or D&D 4e or learned from people who come from those traditions. D&D 5e isn't like those games in many ways. You appear to be conflating a task with an ability check. As I stated in my previous posts a few times, a task and an ability check are not the same thing. A player can choose to have his or her character undertake a task, but a task may or may not involve an ability check. Whether there is an ability check is up to the DM, always, who has several criteria to help him or her determine if one is appropriate.

D1Tremere said:

Your quote about Insight ignores the point. As the skill specifically states, the check decides (not the player, not the DM) what your character determines. The part you emphasize just details in game fluff that explains WHY/HOW the check determines. You also avoided my question. If the check is your observation of the environment, what else would determine your thoughts if not that?

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Yes, the character determines that the NPC is exhibiting body language, speech habits, and changes in mannerisms that indicate untruthfulness. That is not telling the player how the character thinks or how it must act.

I did not avoid your previous question - it's just irrelevant. How I would decide to have my character act in the face of the outcome of a task to determine truthfulness isn't important. (Remember, a check is not a task, nor a task a check.) What's important in this discussion is who gets to decide how the character acts and that is always the player, short of magical compulsion.

D1Tremere said:

I would also argue that the rules serve both the DM and the Players as "As the player who creates the game world and the adventures that take place within it, the DM is a natural fit to take on the referee role. As a referee, the DM acts as a mediator between the rules and the players."
Your reference to the rules serving the DM and not the other way around is used in the rulebooks to say that the DM doesn't serve the rules, an explanation that they need not be slavish to them. It does not refer to the players relationships to the rules. If the DM is a referee that would suggest that the rules exist between themselves and the players.

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The players have no relationship to the rules. They can only describe what they want to do. The DM then decides whether any rules apply to resolve those actions. Players might naturally act with the expectation that certain rules may apply (such as having your high-Strength character step up to engage in tasks for which a high Strength may be of benefit), but it's up to the DM ultimately.

D1Tremere said:

I'm tempted to add a snarky comment here to address yours, but I honestly enjoy discussing my perspective with others and see no need to make it less fun.

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I didn't make any snarky comments, and I'm glad you won't either.

D1Tremere said:

"Ability Checks An ability check is a test to see whether a character succeeds at a task that he or she has decided to attempt." Says right here that the player can decide to attempt an ability check whenever they want to see if they succeed or fail at a task. Says nothing about the DM making that decision.

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Wait wait wait....

How did you get from “task” to “ability check”? A task is merely something that a character attempts to do, as described by the player. “I’ll unscrew the nut from the bolt” is deciding to undertake a task. It may or may not require an ability check.

Furthermore, while a box wrench is used to turn a bolt or nut, not all bolts or nuts need or even accept box wrenches. Similarly, an ability check IS used to resolve a task, but not all tasks are resolved using ability checks.

When you combine the passage you quoted with the passages that iserith keeps citing it becomes more clear.

(I will, however, suggest that the troll king demanding an answer from the fighter does not count as the fighter “deciding to attempt a task”.)