He who knows other men is discerning; he who knows himself is intelligent. He who overcomes others is strong; he who overcomes himself is mighty. He who is satisfied with his lot is rich; he who goes on acting with energy has a (firm) will.
Chapter 33 in James Legge’s 1891 translation. Legge renders the Chinese 有力 (“has force”) as “strong” and 強 (“strong/firm”) as “mighty.”
✓Public domain
The Chinese original has been in the public domain for over two millennia, and this specific English wording comes from James Legge’s 1891 translation (Sacred Books of the East, Vol. 39), first published well before 1931 — so it too is public domain in the United States. Modern translations of Chapter 33 (e.g. Stephen Mitchell’s 1988 “Mastering others is strength; mastering yourself is true power”) remain under copyright, but that is not the wording quoted here.
⚠ This reflects the original work, which is public domain. If your exact English wording comes from a specific modern translation, that translation may itself still be under copyright — check the edition you quote.