Mens sana in corpore sano is a famous Latin quotation from Juvenal (Satire X line 356).
It translates as "A sound mind in a sound body." The loosely-translated meaning in English for the phrase is "A healthy mind in a healthy body." Over time, the phrase has come to mean that only a healthy body can produce or sustain a healthy mind.
The phrase is taken from a longer sentence, "Orandum est ut sit mens sana in corpore sano," which may be translated: "You should pray for a healthy mind in a healthy body," which is a very different thing than the current meaning above.
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