I was referring to the rules of D&D, not the table rule of how we handle PCs harming or hindering each other. The rules of D&D are adjudicated impartially, when the DM decides they come into play.
Here's my takeaway from this. You're just fine in limiting player action declarations for things you don't like. In this case, you don't like inter-player conflict, so you've instituted a table rule that shorts out the normal game rules and declares that narrative power moves from the DM to one of the players involved (I'm unclear as to how you determine the "defending" player, is it just a matter of who declares first?). You do this because you wish to restrict a type of interaction that you find doesn't suit the type of game you wish to run. I hope I have that largely correct?
Yet, you're arguing that others should not do a similar thing in cases where you don't think it's appropriate. The difference isn't that we disagree that you can't limit character action declarations or resolutions according to the personal tastes of the DM, but that you don't think doing so with stats is appropriate. I'm sure there's an argument to be made that a class of action declarations that relates to who it's targeted at is someone significantly different from a class of action declarations based on a stat, but that's special pleading.
That I find a particular play experience awful doesn't mean anything other than I find it awful. For me. It's not a claim that it is objectively awful.
I will endeavor to recall this. I'm sure you extend the same courtesy to people that tell you that your choices in running are awful?
I've seen and experienced plenty of conflicts that were not well-received. Plenty of horror stories about this sort of thing on the forums throughout the years as well.
Sure, which is probably why you've decided to just close off the ability of your players to engage in that type of action.
Whoa, deja vu. I'm almost positive I've said almost that exact thing to you before. Have I?
Anyway. I've also seen horror stories. That comes from no controls or a player that's not working out well with the group dynamic. However, it's also perfectly good to set expectations at the beginning of play and enforce them that can ensure that the abuses and horror stories are unlikely to occur (anything involving people is uncertain) but still allows for good inter-player conflict. An example: in my current campaign, the requirements for PCs are, "play someone who will care about doing heroic deed," and "play a character that will work well in a group." As we're all friends, this is easy, but I made sure to say it anyway to set the tone of the campaign. I've had to once work with a character concept that was going to have trouble with those two requirements, and did so by asking the player how his character was going to meet those requirements. A few changes later, and it was good, so it does work.
In play, a few conflicts have arisen. The biggest one, and one that I'm enjoying the most, deals with the main plot. A short summation is that the Dwarves long ago had a massive empire, but then blew up the world. How, or with what, is not known today. It took many hundreds of years for civilization to begin to recover and much knowledge was lost. Okay, a tad trite, but there's a twist. The players uncovered a plot by a group of well funded and powerful cultists to acquire the knowledge the dwarves used to effect the cataclysm. The players have been racing to gather the knowledge first and keep it from the cultists. However, in their journeys, they've also begun to run across a strange kind of crystalline growth that warps the surroundings into nightmare landscapes and horrid aberrations. They figured out that the two things may be related, and sought more information. They uncovered that this rot had occurred before, in the time of the Dwarven Empire, and had grown out of control of even the power of that era. The 'cataclysm' was an attempt by Dwarves to use an even more ancient power to fight the rot; a weapon tied to the forces of creation. They can't tell if it worked properly or if the dwarves did something wrong and also caused the cataclysm, but either way, it seemed to stop the rot.
So, the conflict? Well, the party had loosely organized into three camps. Those that think that it may be necessary to 'push the button' and use the weapon again to stop the rot, and so wish to acquire the weapon for that purpose. Those that think that the weapon is clearly flawed, and that it should be acquired to prevent it's use at all costs while another solution to the rot is found. And a third, neutral faction that hasn't decided. Both the 'push the button' and the 'no nukes' camps are quietly making contingency plans to ensure that they can 'do the right thing' if push comes to shove, but are still working with the other group and attempting to use argument to convince the other side and/or more of the unaligned group to join their side.
If push does come to shove, and they fall into fighting each other over what's the 'right' way to save the world, I wouldn't step in to prevent them from playing the characters they want. My players understand the difference between conflict between characters and conflict between players, and, in this case, the conflict is clearly between the characters. I wouldn't even begin to know who to select as the 'defending player' allowed to dictate the outcome to the other players in this case.
At the time, I understood you to say that in order to solve the "problem" of players acting smarter than you believe their character's Intelligence would allow (players you didn't know well because the one you do wouldn't do that, natch), you would call for an ability check. I took that to mean that playing at a table with such players would mean you would use that solution unilaterally going forward. If so, I think that solution produces other problems. If that's not a solution you would apply unilaterally, then disregard.
So disregarded.
I don't think you have answered the question as to what fictional circumstance is making turning the dial to "S" uncertain enough to warrant an ability check.
You're right, in that you're fishing for a hard, concrete answer when my answer all along has been, 'it's like porn, I know it when I see it.' The number of considerations that would go into that decision are myriad and defy easy quantification. I'm sure you don't run your game with a concrete list of 'when I see 'X' I will then do 'Y'' statements, so why are you requiring me to provide you with such a declaration, especially when I've clearly conceded that it's a matter of judgement in specific cases?