Why the reasonable moon produces 'lunatics'
the moon is the mother of lunatics and has given to them all her name
The ending turns an etymology into a paradox. Lunatic comes through French or directly from Late Latin lunaticus, meaning moon-struck, from luna, moon. Its earlier English sense associated recurrent mental disturbance with changes of the moon; its later use broadened beyond that explanation. Douglas Harper, Online Etymology Dictionary, 'lunatic', adjective and noun entries.
Chesterton exploits the inherited association while reversing its expected lesson. The heavenly body whose outline seems clearest and whose recurrence seems most predictable becomes his emblem of reason trapped in a circuit. The word history supports the final pun; it does not establish that the moon causes mental illness, or that lucid reasoning does.
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