By Rabbi Gil Student

Imagine that medical science advances to the point where doctors can replace a human brain with an artificial one.

The replacement is an extraordinarily sophisticated computer that performs every function of the original brain. It uploads and preserves the patient’s memories, personality and intellectual abilities. After the operation, he recognizes his family, continues his work and behaves exactly as he did before. Is he still the same person? This possibility, one form of transhumanism, raises an important question about the nature of human identity. As artificial organs become increasingly sophisticated, we can imagine replacing a heart, lungs, limbs and eventually even the brain. At what point would a human being cease to be human and become a machine? I suggest that Jewish tradition offers a way to approach this question by distinguishing between the spiritual soul and the physical body and organs through which it operates.

I. The Soul Fills the Body

The Gemara (Berachos 10a) discusses five occasions when King David says, “Bless the Lord, my soul” (Ps. 104:1). The Sages explain that David offered these five praises because of five similarities between G-d and the human soul. Just as G-d fills the entire world, the soul fills the entire body. Both see without being seen, sustain what they inhabit, are pure, and dwell in the innermost chambers. The Gemara concludes that the soul, which possesses these characteristics, should praise G-d, Who likewise possesses them. For our purposes, this particular comparison is important: G-d is present throughout the universe, yet no physical object contains Him. His presence does not make Him part of the physical substance of the universe. Similarly, the soul fills and sustains the body, but is not contained in any specific organ or even the entire body. The soul exists outside of the body which it fills.

Rabbenu Bachya ben Asher (14th cen., Spain) quotes this Gemara and explains that just like G-d’s presence is particularly strong in certain places like Mt. Sinai and Jerusalem, He still can be found anywhere in the world. Similarly, the soul manifests itself particularly in the brain, heart and tongue but nevertheless fills the entire body, and every organ contains some manifestation of its power. Despite that presence in the body, the soul is separate from it. Any defects in the body do not exist in the pure soul because it is separate from the body (Kad HaKemach, Avel, section 3).

Rav Yitzchak Arama (15th cen., Italy) similarly explains the Gemara’s comparison between G-d and the soul. The soul’s existence is evident because its influence extends throughout the body, yet its essence cannot be perceived through the physical senses. Even the Gemara’s statement that the soul dwells in the innermost chambers refers to the difficulty of understanding its nature. The soul is known through its effects, while its essential nature remains hidden from human understanding (Akeidas Yitzchak, Emor, no. 68).

II. The Body as an Instrument

Rav Meir Leibush Weiser (Malbim; 19th cen., Ukraine) develops this distinction in Artzos Ha-Shalom (derush 3), a discussion he revisits in his commentary to Song of Songs (3:10). He describes the soul’s powers as constituting the essence of the human being, while the body and its organs serve as garments through which those powers operate. The intellectual soul has its principal dwelling place in the head, while other faculties are associated with the heart and lower organs. Malbim explains this further in his commentary to the Torah (Gen. 2:7). The higher soul is one in its essence, but it manifests different powers according to the physical instruments through which it operates. He associates intellectual activity with the brain, vitality with the heart and nourishment with the lower organs. The manifestation of these powers depends upon the condition of the physical instruments.

Returning to the case of an artificial brain, if the natural brain is an instrument through which the soul exercises its intellectual powers, replacing that instrument need not mean replacing the soul. The artificial brain allows the person to think, remember and communicate, just as an artificial heart allows blood to circulate. The spiritual identity of the person remains distinct from the machinery that makes these activities possible.

III. The Soul Beyond Matter

Rambam (12th cen., Egypt) distinguishes between the spiritual soul and the faculties that depend upon the physical body. In Moreh Nevuchim (part 1, ch.

70), he explains that the word “soul” has different meanings. The soul that animates a person at birth is a faculty of life, while the soul that survives death has an independent existence. He similarly distinguishes the ability to think, which operates through the body, from the acquired intellect, which can exist independently of it (ibid., ch. 72).

Rambam likewise expresses this distinction in Mishneh Torah (Hilchos Yesodei HaTorah 4:9), where he describes the higher intellectual form of the human soul as independent of physical matter: “This form of the soul is not composed of the elements.” It comes from G-d and does not perish when the body decomposes. Similarly, later in Mishneh Torah (Hilchos Teshuvah 8:2– 3), Rambam explains that in the World to Come, the souls of the righteous exist without physical bodies. The enduring spiritual soul therefore does not require the continued existence of the brain or any other organ.

From a kabbalistic perspective, Rav Chaim Volozhiner (19th cen., Belarus) distinguishes three levels of the soul, commonly called nefesh, ruach and neshamah. These represent different dimensions of spiritual life and have different relationships with the physical body (Nefesh HaChaim, sec. 1, ch. 15). He explains their relationship through the analogy of a glassblower. The breath passes from the glassblower’s mouth through a tube into molten glass. The lowest spiritual level, nefesh, resides within the body, while ruach connects the body with a higher spiritual source. The essential neshamah, however, ordinarily remains beyond the body. Its actual essence does not enter the human body at all, although its spiritual influence extends into the person.

Rav Chaim further explains that references to the soul residing in the brain concern the radiance of the neshamah that illuminates the person’s brain and intellect. They do not refer to the essence of the neshamah itself, which remains outside the body. It follows that the replacement of a biological brain with an artificial one would not necessarily remove the soul, because the soul was never physically contained within the original brain.

IV. Would the Person Remain Human?

Returning to our hypothetical patient with an artificial brain, he has an artificial system that preserves his memories, personality and ability to think. He continues to recognize his family, fulfill his responsibilities and behave as he always did. Has he become a machine because his brain is now a machine? The soul is spiritual, while the brain is an organ through which some of the soul’s powers are expressed. If a person’s essential soul remains associated with him, replacing the brain would not, by itself, eliminate his humanity. However, when a person dies, his soul leaves the body with which it was associated. The Torah describes Rachel’s death while giving birth to Binyamin: “And it came to pass, as her soul was departing, for she died” (Gen. 35:18). The Gemara (Shabbos 152b) describes how after a person dies, his soul ascends and descends for twelve months and then permanently leaves earth.

Therefore, the association of a soul with a body depends on whether the person is alive. The halachic definition of death is a hotly debated subject that need not detain us here. Ultimately, the status of someone with an artificial brain depends on whether that person remains alive or becomes the walking dead.