IPA: /ˈθɪəraɪzər/
KK: /ˈθiːəraɪzər/
A person who creates theories or ideas about how things work, often based on speculation or reasoning.
The scientist is a well-known theorizer in the field of quantum physics.
The word 'theorizer' is formed from 'theory' (from Greek 'theoria', meaning contemplation or speculation) and '-izer' (a suffix used to indicate a person who performs a specific action). Thus, a theorizer is a person who engages in the act of forming theories or speculating about concepts.
Think of someone who is 'contemplating' ('theoria') and then 'doing' something ('-izer') — that's why a theorizer is someone who creates or discusses theories.