also: げんゆう
noun
A somewhat formal or literary word for the underlying reason or cause of something. Often used in explanations or investigations.
事故の原由を調査する。
Investigate the cause of the accident.
彼が辞職した原由は明らかではない。
The reason for his resignation is not clear.
Compound of 原 (origin, source) and 由 (reason, cause). The exact historical derivation is uncertain, but the combination naturally conveys 'original reason'.