This question has been debated for thousands of years. The Torah establishes that the embryo begins to take form 40 days after conception, but there are varying opinions when the soul enters the body.

From Conception

The Talmud records a debate between the Roman Emperor Antoninus and his friend Rabbi Judah the Prince, redactor of the Mishnah:

Antoninus said to Rabbi Judah: From when is the soul placed in a person? Is it from the moment of conception or from the moment of the formation [of the embryo, which happens 40 days later]?

Rabbi Judah replied that it happens at the time of formation.

Antoninus challenged the assertion: Can an unsalted piece of meat last for even three days without rotting? [Surely an embryo could not exist for 40 days without a soul!] Rather, [the soul must be present] from the moment of conception!

Rabbi Judah conceded: Antoninus taught me this matter, and Scripture1 even supports him . . .2

Time of Formation

Elsewhere, the Talmud states without any debate that the soul enters at the time of formation:

Rabbi Yochanan and Rabbi Elazar both say: The Torah was given in 40 days [that Moses spent atop Mount Sinai] and the soul is formed in 40 days.3

Long After Birth

The mystics give a much later time frame for the soul’s entrance. They say that the soul begins an early phase of entry at the time of brit milah4 (or for a girl, at birth or baby naming5) and more so at the age of chinuch when the child starts becoming more accustomed to following Torah and mitzvahs.6

Additionally, the Zohar states that the soul’s entry into a man’s body is completed when he reaches age 13 years and a day, and into a woman’s body when she reaches age 12.7

The Midrash Rabbah proposes that there are different phases for the entry of the soul, based on the verse in Ecclesiastes, “Better a poor and wise child than an old and foolish king . . .”8 The interprets the “old and foolish king” as the yetzer harah, the evil inclination or “animalistic soul,” since it has been with the person since birth, while the yetzer tov, G‑dly inclination, is called a “wise child,” as it only (fully) enters at the age of bar or bat mitzvah.9

Two Souls? Torah in Utero

Based on the Midrash, it is tempting to reconcile the differing statements by explaining that there are multiple things referred to as a “soul.”

There is the soul that simply converts a person from a dead hunk of meat into a living, breathing and thinking organism, referred to as nefesh hachiyunit (“life-soul”) or nefesh habehamit (“animal soul”). This is the soul referred to by Rabbi Judah and Antoninus, who debated whether it enters at conception or when the embryo begins to take form.

And then there is the nefesh Elokit (“G‑dly soul”), which we are told enters at the time of circumcision or when the baby is given her Hebrew name.

However, it appears that the G‑dly soul is already present in some way in the womb, as the Talmud states that each child is taught the entire Torah while still in utero.10

Presumably, this activity would be done by a G‑dly soul, leaving us to reconcile the many contradictory statements about the soul’s entrance.

Unity of Body and Soul

How do you actually define “the soul entering the body”? The Rebbe explains11 that the soul and body fuse to the point where they become like one entity, and the influence of the soul is apparent on the body.

So although the fetus has a G‑dly soul, the soul’s influence is limited, as its connection to the body is weak. In fact, even the animal soul is not yet exerting its full life-giving influence, as we can see clearly that the fetus is nourished by the mother, not its own breath, etc.

Upon birth, what is readily recognizable is that the animal soul has become fully manifest, as the child is an independent entity. However, the G‑dly soul, which is there but in a more transcendent fashion, has yet to express its influence, and the child’s actions do not necessarily reflect its presence.

At the circumcision or naming, the G‑dly soul begins to become internalized within the consciousness of the person. This process becomes complete and manifest at bar or bat mitzvah, when one is empowered (and thereby obligated) to observe the mitzvot.