“How Can I Know What I Think Till I See What I Say?” (1)

This is possible if the person has a thinking (and talking or writing) alternate personality with whom the regular personality is not co-conscious.

See also (2).  This is possible if the regular personality hears an alternate personality as a voice in its head.

1. Quote Investigator:  https://quoteinvestigator.com/2019/12/11/know-say/ 

2. The Guardian: “Majority of authors ‘hear’ their characters speak, finds study.”

https://www.theguardian.com/books/2020/apr/27/majority-of-authors-hear-their-characters-speak-finds-study