The Basics of Judaism: G-d & Torah: The Soul and the Body:
When does the soul enter the body? Where is it until then?
According to the Talmud (Yevamos 62a), unborn souls are "stored"
in a place called GUF (not to be confused with the usual translation
of "GUF" as "body"). The souls study Torah, but they forget it when
they are born. (If this applies to non-Jewish children, they must
study the Noahide Laws.) The soul enters the body at conception
(Talmud Sanhedrin 91b).
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