“The cruelest prison of all is the prison of the mind.”

Voltaire (1916)
Context: Voltaire was not the first or last man to convert a prison into a hall of fame. A prison is confining to the body, but whether it affects the mind, depends entirely upon the mind.
It was while in prison that he changed his name from the one his father gave him — Arouet — to the one he has made famous throughout all time — Voltaire. He said, "I was very unlucky under my first name. I want to see if this one will succeed any better."

Variant: Happiness is the most insidious prison of all.
Source: V for Vendetta (1989)

Source: Bloodline

Source: Simone Weil : An Anthology (1986), Human Personality (1943), p. 69
Context: If a captive mind is unaware of being in prison, it is living in error. If it has recognized the fact, even for the tenth of a second, and then quickly forgotten it in order to avoid suffering, it is living in falsehood. Men of the most brilliant intelligence can be born, live and die in error and falsehood. In them, intelligence is neither a good, nor even an asset. The difference between more or less intelligent men is like the difference between criminals condemned to life imprisonment in smaller or larger cells. The intelligent man who is proud of his intelligence is like a condemned man who is proud of his large cell.

Discipline and Punish (1977) as translated by Alan Sheridan, p. 228
Discipline and Punish (1977)

Source: God’s War (2011), Chapter 4 (p. 42).