“He who is subjected to a field of visibility, and who knows it, assumes responsibility for the constraints of power; he makes them play spontaneously upon himself; he inscribes in himself the power relation in which he simultaneously plays both roles; he becomes the principle of his own subjection.”
Discipline and Punish (1977)
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Michel Foucault 128
French philosopher 1926–1984Related quotes

“Most powerful is he who has himself in his own power.”

Kant, Immanuel (1996), page 100
Anthropology from a Pragmatic Point of View (1798)

Misattributed to Chateaubriand on the internet and even some recently published books, this statement actually originated with L. P. Jacks in Education through Recreation (1932)
Misattributed

“He who controls others may be powerful, but he who has mastered himself is mightier still.”
Variant: He who conquers others is strong; He who conquers himself is mighty.

Appendix VI : A few principal rituals – Liber Reguli.
Magick Book IV : Liber ABA, Part III : Magick in Theory and Practice (1929)