The episode title is taken from the final verse of the poem "To Althea, from Prison" by Richard Lovelace, which Dr Martel was trying to remember all the time that he was in prison: "Stone walls do not a prison make, / Nor iron bars a cage; / Minds innocent and quiet take / That for a hermitage. / If I have freedom in my love, / And in my soul am free, / Angels alone, that soar above, / Enjoy such liberty."