Who said "I am not mad, but a knight errant"?

This quote is attributed to Don Quixote.

I am not mad, but a knight errant - Don Quixote

"I am not mad, but a knight errant"

Don Quixote

Character in Don Quixote

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"The Knight of the Sorrowful Countenance"

"I know who I am, and who I may be, if I choose."

"Facts are the enemy of truth."

"The pen is mightier than the sword."