For who knoweth what is good for man in this 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? Ecclesiastes 6:12 (KJV)
‘… all of the days of his vain life,’ refers to the ‘accusative of time. (Seow, 233) ‘… spendeth as a shadow’ refers to human life being ephemeral, (234) i.e., human life is short and thereby ignorant. The implication is that man is in no condition to judge what is good for himself, in fact, he does not even know what is going to happen to him after death. (Bartholomew, 204)
References:
Bartholomew, C. (2009). Ecclesiastes, Baker Academic
Seow, C.L. (1997). Ecclesiastes, Yale University Press