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I; me
「己」 has other readings with different meanings.
己の力だけでここまで成し遂げたと胸を張った。
He stood proudly, having accomplished this much on his own.
己を過信すると思わぬ失敗を招くことがある。
Overestimating oneself can lead to unexpected failures.
まずは己の弱点を認めることから始めよう。
Let us start by acknowledging our own weaknesses.