Sure I believe it, because the bible proclaims it.
Ez.18:3As I live, saith the Lord GOD, ye shall not have occasion any more to use this proverb in Israel.
4Behold, all souls are mine; as the soul of the father, so also the soul of the son is mine: the soul that sinneth, it shall die.
Psa 146:4 His breath goeth forth, he returneth to his earth; in that very day his thoughts perish.
Ecc 9:5 For the living know that they shall die: but the dead know not any thing, neither have they any more a reward; for the memory of them is forgotten.
And Jesus proclaims it.
Jn.11: 11These things said he: and after that he saith unto them, Our friend Lazarus sleepeth; but I go, that I may awake him out of sleep.
12Then said his disciples, Lord, if he sleep, he shall do well.
13Howbeit Jesus spake of his death: but they thought that he had spoken of taking of rest in sleep.
14Then said Jesus unto them plainly, Lazarus is dead.