What defines a PC's consciousness of self, memories and knowledge?

Question

I am not sure whether a PC's consciousness, memories and knowledge are related to their personality or their mental stats (in particular, INT and WIS). My doubt comes from a practical situation, that is, a wizard that casts True Polymorph or Shapechange on himself, becoming another intelligent creature. With True Polymorph the mental stats are replaced, while the alignment and personality are retained. With Shapechange, the mental stats are retained too.

The True Polymorph spell description say:

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 Shapechange spell description says:

Your game statistics are replaced by the statistics of the chosen creature, though you retain your alignment and Intelligence, Wisdom, and Charisma scores.

The spells' descriptions do not say anything about the creature's memories and prior knowledge. While it seems quite obvious that a PC (or other character or monster) that casts Shapechange remembers who he is, the same cannot be said for True Polymorph. I think that the manuals don't say anything about subject's memories. Are they retained or changed?

Answer 1

Consciousness, memories and knowledge are determined by personal characteristics, not abilities.

In the description of spell Feeblemind, Player's Handbook pages 239/240 you can read that:

On a failed save, the creature’s Intelligence and Charisma scores become 1. The creature can’t cast spells, activate magic items, understand language, or communicate in any intelligible way. The creature can, however, identify its friends, follow them, and even protect them.

In the description of another spell, that is Modify Memory:

A modified memory doesn’t necessarily affect how a creature behaves, particularly if the memory contradicts the creature’s natural inclinations, alignment, or beliefs. An illogical modified memory, such as implanting a memory of how much the creature enjoyed dousing itself in acid, is dismissed, perhaps as a bad dream. The DM might deem a modified memory too nonsensical to affect a creature in a significant manner.

According to page 177, Player's Handbook:

Intelligence Checks An Intelligence check comes into play when you need to draw on logic, education, memory, or deductive reasoning.

If you want to remember how exactly many freckles the redhead, that you have seen one month ago in a crowd of one thousand people, had on her face it will have very hard or nearly impossible difficulty (DC 25 or 30). However if you want to remember the most possibly simple thing, that is a context of a memory lasting no more then 10 seconds, it has very easy difficulty (DC 5). It means that even with your Intelligence of 1(-5 modifier), you can remember that thing, because your passive Intelligence check is 5 and it is just enough.

As you can see, abilities do not determine your tendencies. Even if some of yours memories could state something absurd, you have your alignment and ideals, which will tell you what is right to do(or maybe not right, if you are a bad person). However if something changed your alignment, it could make you act differently, like even attacking your teammates.

Everything is stated in the Player's Handbook, Chapter 4: Personality and Background. Pages 121-141.

Answer 2

Personality is related to the preference of actions. If a character wouldn't attack a loved one when they had an INT of 12, they wouldn't attack them with an INT of 3.

The difference comes with how they wouldn't attack. The character with INT 12 would recognize the loved one and choose not to attack. The character with INT 3 would hesitate and then have to roll for it to remember the loved one and not attack. The higher INT allows for automatic choice. With a low INT, some options are restricted.

I had a character turned into a frog once. He was still able to stay near the party instead of wandering off for random food, but I had to roll a Will save at the time. I also had a character that turned into a mind flayer. I had to roll will to not perform evil deeds.