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Mine is probably:

The mind commands the body and it obeys. The mind orders itself and meets resistance.

I'm partial to the litany against fear. Not super deep, but I can say I've recited it quietly to myself a few times! It's a decent tool to acknowledge, process, then repress and move past true fear.

I must not fear. Fear is the mind-killer. Fear is the little-death that brings total obliteration. I will face my fear. I will permit it to pass over me and through me. And when it has gone past, I will turn the inner eye to see its path. Where the fear has gone there will be nothing. Only I will remain

Not as a quote, but as a line in the context of the book, "They tried to kill my son" repeated over and over was a great illustration of a rational, quite staid man fighting a building rage and becoming something different.

It also served to humanise the Duke right before the tooth scene, building an empathetic connection to make us care about that scene more.

A deceptively simple line used incredibly well.

For pure quotage "It is by will alone I set my mind in motion".