The Republic by Plato Part 6 Page 47

(Heraclitus said that the sun was extinguished every evening and relighted every morning.)

But what ought to be their course?

Just the opposite. In childhood and youth their study, and what philosophy they learn, should be suited to their tender years: during this period while they are growing up towards manhood, the chief and special care should be given to their bodies that they may have them to use in the service of philosophy; as life advances and the intellect begins to mature, let them increase the gymnastics of the soul; but when the strength of our citizens fails and is past civil and military duties, then let them range at will and engage in no serious labour, as we intend them to live happily here, and to crown this life with a similar happiness in another.