That phrase comes from the poem "To Althea, from Prison" written by Richard Lovelace in 1642, while he was imprisoned. There are four stanzas of the poem, and the phrase that appears in Tuck Everlasting comes from the final stanza:
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.
In Natalie Babbit's novel, the phrase becomes Winnie's mantra as she helps the Tucks break Mae out of jail. At first, Winnie says "the lines repeated themselves in her head till they were altogether meaningless," but as Miles pries the nails out, Winnie "counted carefully, while behind her counting, her mind sang, 'Stone walls do not a prison make.'" Repeating the lines in her head gives her courage, so she thinks of them while they dismantle the physical prison holding Mae.
The words have another meaning as well: in the context of the poem, Lovelace uses these...
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