What is and is not “personality” for the purpose of Polymorph?

Question

It's well understood that "___ Polymorph" spells overwrite pretty much everything in a stat block (or character sheet) with numerical functionality, including languages, proficiencies, and named features. Everything except "alignment and personality".

What does "personality" mean? A very inclusive option would be "any knowledge or memory that doesn't have a predefined quantified effect". A very exclusive option would be "personality traits, flaws, bonds, & ideals exactly as written, plus relationships & emotions, but otherwise tabula rasa as befitting a creature who just sprang into existence".

As pointed out, "What defines a PC's consciousness of self, memories and knowledge?" is indeed a similar question, so I'll refocus this one.

Is there rules evidence that the original creature's knowledge and memories are retained after a Polymorph, other than emotional responses about people/places/objects/actions they like or dislike (aka "personality")?

Let's look at this example creature named "Bob":

• Pre-Polymorph Bob ("PRE") was an AL-legal medium-sized humanoid PC with 10 Intelligence, proficient in History, Nature, Performance, Violin, Thieves Tools, Common, Halfling, all Armor, Simple and Martial Weapons, able to cast Sorcerer spells up to 3rd level.

• True Polymorphed Bob ("POST") is a medium-sized non-humanoid creature that includes the complete humanoid body plan (possibly plus wings, a tail, or other bonus parts) with 10 Intelligence, proficient in Stealth, Perception, Tinkers Tools, Common, Gnomish, Light Armor, Simple Weapons, no spellcasting.

Unlike Feeblemind or Modify Memory (the examples in the prior question's answer), Polymorph changes the creatures proficiencies. Obviously, POST is not proficient at the Charisma (Performance) or Dexterity (Violin) checks needed to put on a show or play a particularly difficult piece. But more importantly, POST also is not proficient at Intelligence (Performance) or Intelligence (Violin) checks to answer informational questions about the process of a violin performance. POST is no better at this task than an average Commoner. POST is missing some portion of PRE's knowledge. Furthermore, POST has gained knowledge (languages, skills, tools) that PRE never possessed.

If we need to use Magi-Babble to say that somehow Bob can lose and gain proficiencies without any change to their memories, then okay, I'll accept that, but I'd like to have rules justification for it. Otherwise it makes more sense to me that POST is actually a completely new being that's strongly influenced by the presence of PRE's "soul".

This link is somewhat helpful, but Jeremy Crawford only talks about "identity" and "core personality" in terms of caring about your friends, likes vs dislikes, and other emotional responses. Unfortunately he doesn't discuss memories and factual knowledge.

Pre-Bob: Hello, I'm Bob the violinist!

Alice: Oh? What do you like about violins?

Pre-Bob: Wow, let me tell you, there are so many things... blah blah blah...

(15 minutes later)

Alice: Sheesh, stop already! TRUE POLYMORPH!

(POP)

Post-Bob: I'm still Bob the violinist!

Alice: Oh? What do you like about violins?

Post-Bob: ... I don't know. I just like them, okay?

Answer 1

It's largely up to DM's interpretation.

"Personality" isn't a game term per se. It retains the ordinary meaning of that word and is adjudicated by the individual DMs in the event that a question arises.

The closest thing to a definition is in PHB p. 123, Personal Characteristics, which states:

Fleshing out your character's personality—the array of traits, mannerisms, habits, beliefs, and flaws that give a person a unique identity—will help you bring him or her to life as you play the game. Four categories of characteristics are presented here: personality traits, ideals, bonds, and flaws. Beyond those categories, think about your character's favorite words or phrases, tics and habitual gestures, vices and pet peeves, and whatever else you can imagine.

That last part is important, because it asserts that almost anything, other than game statistics or physical traits, could be considered part of a character's personality.

The text of true polymorph defines how a character changes when affected by that spell:

The target's game statistics, including mental ability scores, are replaced by the statistics of the new form. It retains its alignment and personality.

... The creature is limited in the actions it can perform by the nature of its new form, and it can't speak, cast spells, or take any other action that requires hands or speech unless its new form is capable of such actions.

The spell doesn't say that you lose your memories (only your mental ability scores), so there's no reason why you would forget things you knew, or things you knew how to do. You're still you in every non-physical way, but the new form's brain may limit you—think of that one Quantum Leap episode where Sam jumps into the body of a mentally disabled man and finds his mental faculties are likewise impaired.

I would also interpret the last line of the quote above to mean that the physical form is what limits your actions. A human polymorphed into a dog doesn't forget how to use a sword, they just lack hands.

In short, at least as I interpret it—your DM may indeed interpret it otherwise:

• Any game statistics of the old form are replaced
• Your mental state is retained (except for mental ability scores), so that you retain all of your old knowledge, personality, memory, and so on
• You lose things that you no longer have the physicality to do
• For things on the borderline between personality and game statistics (e.g. proficiencies), it's up to the DM. Polymorph was at least this absurdly complicated in D&D 3rd edition too, and we won't see a stricter definition of what you gain and lose while polymorphed unless you want the spell description to be a whole page long.

Jeremy Crawford gives his take on these rules in Dragon Talk: Sage Advice on Polymorph, 10/2/17. His interpretation is that you literally just replace your character sheet with the monster statblock. You lose your class features, feats, spellcasting, and whatever else is on your character sheet, retaining only your inherent identity and alignment.

Answer 2

The inclusive option - it refers to everything about the creature that polymorph doesn’t overwrite

There are two reasons for this:

1. The definition of personality: “the combination of characteristics or qualities that form an individual's distinctive character.” That is, everything about you that makes you you as opposed to anyone else.
2. Spells do what they say they do. Polymorph tells you what changes explicitly and lumps everything else in as “alignment and personality” (although, from above the former is a subset of the latter).