This is the 4540th most frequent Japanese word.
己
Self; oneself.
"己" in this context is used to denote possession, specifically one's own desires or feelings.
己の欲望を制するのは難しい。
It is difficult to restrain one's desires.
In this sentence, "己" refers to "oneself" or "self," used to indicate self-awareness.
己を知ることが成功への第一歩です。
Knowing oneself is the first step toward success.
Here, "己" means "own" or "personal," signifying possession or acknowledgment.