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When you apply your spell list to... other situations.
(ttrpg.network)
Going through the spell list, there's an awful lot of spells that are incredibly funny in the context of bedroom pastimes, here, let's assume it's heroism and Bear's Endurance.
Maybe it's a 5e thing. I only got into DnD in 4e when I was a kid, so maybe that was the transition point for that. As long as I have been playing, wisdom was for avoiding/deflecting psychic effects whereas Charisma was for withstanding effects that can just hit you with no chance of escape. It's analogous to making dex save to dodge a poison dart trap, but having to make a Con save to resist the poison itself.
The way I have been exposed to it, you use Intelligence to see through an illusion, Wisdom to avoid being charmed, and Charisma to avoid being NPCified into a mindless thrall of the big bad.
I'm afraid this isn't really accurate either. Most of the saves to avoid "being NPCified" (e.g. Vampire charm, Aboleth mind control, dominate person) are wisdom saves.
In 5e, it's safe to assume the default mental save is Wisdom. 90% of mental save effects target Wisdom, and if you're wondering which mental stat to buy resilience for, Wisdom is a clear winner.
If it's a "figure something out" effect or a contest of intelligence, it'll often an INT test, but those are rare. Good examples are piercing illusions, or things like trying to mentally force someone out of your minds (the detect thoughts intelligence contest)
Charisma tends to also be rare, but which things are CHA saves are all over the place, almost arbitrary. Effects that require a mental save but would have a physically debilitating effect (such as divine word, or the jumpscare attack of ghosts) are sometimes Charisma, and those are the most common appearances of it. Feeblemind, which drains your intelligence, is a charisma save for some weird reason. Zone of Truth is a charisma save...
It's not really well defined metaphysically, and I'm not sure there's any directed intention in the design plan here (at least not that I can tell.) - Charisma saves do commonly mean the effect is going to be really bad though...
I have been thinking about exactly this, because the more I think about it the more I wonder why all the DMs I've played with have come to the exact same conclusions when they have to improvise something that doesn't follow a monster stat block or a spell description. I think it comes down to how in Planescape when you are astral projecting, you substitute your Str for Int, Dex for Wis, and Con for Cha.
For the record, I had no idea that this was a 2nd edition thing. I thought that this was just how Astral Projection worked! So, it seems that the DMs and who showed me how DnD is done were old-school players who internalized this into a broader design philosophy, which became a teaching I learned as obvious fact.
I had to do a lot of googling to try and find where this came from, and I'm not 100% sure I am right, but it is pretty frustrating that the only design guidance I had for this doesn't even come from the text itself.
Throughout all of DnD (although 4e is a bit of an exception to this) - the creators have very much relied on something termed the "Friendly Uncle" method of teaching the game.
That is, you don't really learn to play DnD by opening the rulebooks and reading them end-to-end, you join a campaign with someone experienced (the "friendly uncle"), and they teach you the game as you go. The books serve as a sort of "reference" for common rules and making ideas, but most of your roleplaying experience comes from others.
As such, most groups and players of DnD have lots of conventions and house rules that they may not even realize are house rules. They have explanations for things that aren't in the core books, and they have rules and explanations from older editions - which often defined things in more detail. So much of the game is received wisdom from other players, and derives from someone "making it up" a decade ago when they couldn't find or didn't know an answer.
In one regard this is pretty good, the game is a living entity, constantly evolving, and allowing players with vastly different preferences to enjoy what is (ostensibly) the "same" game.
In the other regard, it's really frustrating when it comes to actually pinning down the mechanics or understanding of anything.
@ahdok @ThisIsAManWhoKnowsHowToGling this is a really smart observation.