Where does “論” come from?

論 (Japanese) comes from Japanese 統辞論, from Japanese 統辞 — syntax.

論 (Japanese): theory

Definitions

Step	Language	Word	Meaning
1	Japanese	統辞論	Synonym of 統語論 : syntax , syntactics
2	Japanese	統辞	syntax

論 — Etymology, Origin & Word History | Etymology Explorer