כִּ֣י מִֽי־יוֹדֵעַ֩ מַה־טּ֨וֹב לָֽאָדָ֜ם בַּֽחַיִּ֗ים מִסְפַּ֛ר יְמֵי־חַיֵּ֥י הֶבְל֖וֹ וְיַעֲשֵׂ֣ם כַּצֵּ֑ל אֲשֶׁר֙ מִֽי־יַגִּ֣יד לָֽאָדָ֔ם מַה־יִּהְיֶ֥ה אַחֲרָ֖יו תַּ֥חַת הַשָּֽׁמֶשׁ׃
For who knoweth what is good for man in his life, all the days of his vain life which he spendeth as a shadow? for who can tell a man what shall be after him under the sun?
Read Jewish philosophical commentary on Ecclesiastes 6:12: and classical philosophical sources.