己の

one's own

→ 己
phrase Rare #32,163
one's own A literary or formal expression meaning 自分の; often used in serious, philosophical, or classical-sounding writing.
JA: 賢者は己の過ちを認める。
EN: A wise person admits their own mistakes.

己の罪を考えろ

Consider your own sins.

— ウィッチ (2016)

己の慣習でないものを人は野蛮と呼ぶ

People call anything that is not their own custom barbaric.

賢者己の失策から学ぶ

A wise person learns from their mistakes.

他人を己のように扱うという道理

the principle of treating others as you would yourself

— イップ・マン 序章 (2008)