This is false for the 1st ed AD&D PHB and DMG. I've quoted the relevant paragraphs upthread.
Oh, really?
1e PHB:
Intelligence is quite similar to what is currently known as intelligence quotient, but it also includes mnemanic ability, reasoning, and learning ability...
1e DMG:
Intelligence: The intelligence rating roughly corresponds to our modern "IQ" scores. However, it assumes mnemonic, reasoning, and learning ability skills in additional areas outside the written word.
Those says very clearly that int = IQ
They say that INT "is quite similar to" or "roughly corresponds to" IQ, but (i) they don't give any metric for that similarity or rough correspondence, and (ii) they both say that INT also includes/assumes things that IQ doesn't. Similarity to and roughly corresponding to are not identical to or synonyms of equals or even uniformly correlates with.
Yes they do give the metric. It's in the MM where it goes over what intelligence scores are equivalent to. Low, highly, genius, etc.
And you still haven't addressed Moldvay Basic, which has an INT chart which has nothing to say about IQ and is all about literacy and language use.
Why would I address something that doesn't matter? Basic ran concurrently with 1e and 2e. It was not an edition of D&D, but rather a separate game that is also D&D. If it was an edition, 1e would be 2e, 2e would be 3e and so on.
No one is treating INT differently for animals - as for humans, so for them it confers certain bonuses or penalties on checks that involve reasoning, memory etc.
You can't refuse the baboon an IQ test and say that you are treating int the same for both humans and baboons. You aren't.
But INT doesn't exhaust cognitive, linguistic etc abilities in 5e. Sufficient proof of this is that (unlike Moldvay Basic and AD&D) language learning is completely divorced from INT.
There is nothing in 5e that says you cannot have an animal learn language. The MM entries are just for the average animal of that type. The DM can modify them, including skills and language while still being within RAW.
And the most fundamental issue is this: where, in 1st ed AD&D, do the rules say that having a particular INT score must serve as a limit on a player's action declaration for his/her PC? I don't see that written anywhere. And in the only discussion of roleplaying in the book - which is the closing section on SUCCESSFUL ADVENTURES - Gygax assumes that each player will do his/her best to bring his/her own intelligence to bear upon the game. The notion that playing a character means pretending to have the mental abilities of that character is nowhere to be found in the 1st ed AD&D core rules. (Of course the DMG mentions that principle in many places as applying to the GM, but the GM is not playing NPCs/monsters in the way that players are playing their PCs.)
The Feeblemind Spell. The brain becomes that of a moronic child and stays that way. If you are correct, the PC can just run around being brilliant because the player is. If I am right, the PC is a moron and plays the equivalent intelligence of a moron. A moron would be in the 2-4 range.
0 non-intelligent or not ratable
1 animal intelligence
2-4 semi-intelligent
5-7 low intelligence
8-10 average (human) intelligence
11-12 very intelligent
13-14 highly intelligent
15-16 exceptionally intelligent
17-18 genius
19-20 supra-genius
21+ god like
It's also funny you mention the Successful Adventures portion of the PHB
Characters must know each other's strengths and weaknesses, physical and mental, in order to meet the problem posed with the correct character or combination thereof.
Note that it says characters and not players, and then goes on to mention mental strengths and WEAKNESSES in order to meet the problems posed. Low int is a mental weakness.
