It's the word 'sophrosyne' (in Greek, the σωφροσύνη above), which originally comes from the name of a Greek goddess who was 'the spirit of moderation, self-control, temperance, restraint, and discretion' and, so the story goes, one of the good spirits that escaped from Pandora's box when the lid was raised.
Now the word means: healthy-mindedness and from there self-control or moderation guided by knowledge and balance. Does that sound familiar...?
Good old Wikipedia goes on to say that the Roman poet Juvenal later re-interpreted this sentiment in one of his verses as 'mens sana in corpore sano', meaning 'a healthy mind in a healthy body'.
In part, his verse went:
You should pray for a healthy mind in a healthy body.
Ask for a stout heart that has no fear of death,
and deems length of days the least of Nature's gifts
that can endure any kind of toil