origin; root; source; fundamental principle
Refers to the ultimate origin or fundamental principle of something, often in philosophical, religious, or formal contexts. Not an everyday word for 'cause'.
万物の本源を探求する。
Seek the fundamental origin of all things.
The root of this thought lies in ancient Greece.
Compound of 本 (root, origin) and 源 (source, spring). The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning 'fundamental source'.