There's some bit of truth to saying "it's uncertain if the DM decides it is", but indulge me for a moment.
The question I'm trying to ask is how can a DM that is not in control of a PC's thoughts, decide if an NPC or other PC attempting to persuade him is uncertain? Of course the DM is going to ultimately decide if it's uncertain, but I wanna know how a DM is going to make that decision. So, if you were the DM and deciding whether an NPC or other PC attempting to persuade a PC was uncertain give me some examples of what would cause you to call that situation uncertain.
So as you know I work from a clear separation of player and character. There are a few reasons for this.
"
...you as a player determining how your character thinks, acts and talks." has been cited (PHB 181) and dwelt on. The first observation I can make is simple, "
player" is on one side doing the "
determining", and "
character" is on the other doing "
thinks, acts and talks".
A second observation follows directly from that: thinking, acting, and talking are all together on the character side. If I concede that mechanics can mediate how a character acts, then the wording implies that those same mechanics can mediate how a character thinks or talks. I could conjure up special reasons for separating "
thinks" out, but for me that runs into a problem: spells are mechanics and they can mediate how a character thinks. I could then additionally say that with regard to "
thinks", spells are in a special category away from all other game mechanics. For me, the problem with that approach is that there are class and race features (such as Warlock and Eladrin features) that are not spells but that again, can mediate how a character thinks. So I need to extend my special category to include spells, and class and race features. Alternatively, I could say that skills are the special case, and that skill game mechanics, uniquely, can't mediate thinks. I prefer the simpler understanding of the system: game mechanics can mediate character "
thinks, acts, and talks."
So then I think about what "
determining" might mean? For me it fits with the core loop explained a few pages earlier (PHB 181). Players describe what they want their characters to do. The DM never controls that, but players can't describe their characters doing things that aren't possible for them once mediated through the rules, such as a player can't decide that their character flies, without something in game (i.e. mechanical) to enable that flying. It seems that "
determining" is in the sense of both decide and ascertain. Where ascertaining respects the game narrative and mechanics. In summary, I don't separate out "
thinks" from "
acts" or "
talks", nor do I say that spells are a special category of game mechanics in regard to any such separating out, any more than I concede class or race features are. Or skills. Players are on one side, involved in deciding and ascertaining - mediated through the game mechanics - with characters staying firmly in the game world on the other side. Along with their (character) thinking, acting and talking. That does not mean that I see those things as equal in terms of how they should best be mediated.
Anyway, thank you all for a really interesting thread. I feel I've gained a few ideas to bring back to my own group, and had a chance to dig more deeply into this thorny matter, and see how others view it. I don't denigrate those views, even though I don't share them.