Bhagavad Gita, Chapter 17, Verse 16

Serenity of mind, kindliness, silence, self-control, honesty of motive, this is called the mental austerity.

The deepest of the three austerities is named here as happening somewhere no one else can see, inside real, private thought.

Serenity and purity of purpose describe an internal discipline that never has to become visible to count. Real austerity, at this level, is entirely a private matter.