In our Yom Kippur confession in the Amidah, we refer to God as “bochen kelayot,” He examines the kidneys as a way of judging people. The reference is not to a physical exam, but to a divine scan of this internal body part for its intentions and thoughts. Why should this be?

It turns out that in Tanach, the kidneys are viewed as the place of conscience, emotions, desires and wisdom. The brain is not mentioned in Tanach. It seems that the functions that we today assign to the brain, the Tanach assigns to the kidneys. (The rabbinic word for brain, “moach,” does appear once in Tanach, at Job 21:24, but it means marrow of the bone.)

It is not just our liturgy, but the Tanach itself that mentions the kidneys (along with the heart) as organs examined by God when judging someone.

Here are the main relevant verses:

Jer. 11:20: The Lord of hosts Who judges righteously, Who examines the kidneys and heart (“bochen kelayot va-lev”)…

Jer. 17:10: I am the Lord “choker lev, bochen kelayot,” to give every man according to his ways, according to the fruits of his doings.

Jer. 20:12: And the Lord of hosts, bochen tzadik, ro’eh chelayot va-lev…

Ps. 7:10: U-vochen libot u-kelayot Elokim tzadik.”

Ps. 16:7: I will bless the Lord Who has given me counsel; in the night my kidneys instruct me.

Ps. 26:2: Examine me Lord and test me; refine my kidneys and my heart.

Prov. 23:16: My kidneys rejoice when my lips speak with uprightness.

Jer. 12:2: You are near in their mouth but far from their kidneys.

The kidneys are also mentioned in the Torah in the context of sacrifices. (See, e.g., Lev. chaps. 3,4,7, 8 and 9.) Of course, no reason is explicitly given for including them, but perhaps their important role in Tanach’s view of humans is a reason they are included in the sacrifices. Their inclusion is not just a minor side matter (as I had erroneously thought!).

It is interesting to point out that the King James Bible translation made the decision to use the word “reins” instead of “kidneys” in any non-sacrificial context of “kelayot.” (I read that in modern times, the Jewish Publication Society is willing to use “mind” or “conscience” for these!)

In 2010, Rabbi Natan Slifkin wrote a lengthy and well-researched article on our topic: “The Question of the Kidneys’ Counsel.” It is available online. My next few paragraphs are based on his ideas. (I am only summarizing a small portion. The article is also included in his 2020 book: “Rationalism vs. Mysticism.”)

The Talmud takes all those verses about the importance of the kidney literally. For example, we find the following passage at Brachot 61a: “The Rabbis taught: The kidneys advise (יועצות), the heart considers, the tongue articulates, the mouth finishes…”

The immediately preceding statement in the Talmud (also preceded with “The Rabbis taught”) states that the two kidneys had distinct roles: one kidney counsels man to do good and the other counsels him to do evil.

The “Nishmat” prayer that we recite on Shabbat (composed in the period of the Talmud or perhaps thereafter) also deserves mention for giving an important role to the kidneys: “All innards (kerev) and kidneys (chelayot) shall sing praises to Your Name…” (Also, there is also nothing in this section about the brain.)

Medical knowledge in Biblical times was very limited and the view of the heart and kidneys expressed in the Tanach seems to have been roughly consistent with what was thought in the surrounding cultures (i.e., Egypt and Mesopotamia). But by the time of the Rishonim, there was knowledge of the importance of the brain and the more limited role of the kidneys. How did the Rishonim deal with the above Biblical verses?

(As further background, it was only in the fifth century BCE that certain Greek thinkers began to argue for the importance of the brain. Then, for centuries thereafter, the issue was: Was it the heart or the brain that was more important.)

Ibn Ezra makes the suggestion that those references in Nach to the kidneys are only verses of metaphor. Because the kidneys are hidden within the body they can metaphorically represent man’s innermost self (“kinuy le-setarim”). See his comm. to Ps. 7:10. (In his comm. on Ex. 23:25 he writes that the neshama=the chochma is housed in the brain.)

Ramban accepts that the kidneys are the sources of counsel. He explains that the reason the kidneys are included with the sacrifices is that they are the instruments of thought and desire. He was aware that the brain is the seat of the mind. See, for example, Kitvei HaRamban, vol. 1, p. 150 where he writes that the purpose of the tefillin of the head is to be facing the brain, which is the “chariot of the soul.” There was a scientific view at the time that the kidneys were linked to the sexual organs.

Rabbeinu Bachya was aware that the brain has a cognitive function. In order to reconcile this with the Tanach and Talmud, he proposed that thoughts are conceived in the brain but need to descend to the heart and kidneys in order to be transmitted as directions to the body. See his comm. to Gen. 1:27 and 6:6.

R. Judah Moscato (16th cent.) writes that the kidneys remove certain negative elements from the blood and release it as urine. Once the blood is cleansed, the forces that power the intellect will produce elevated and perfected thoughts. (But he does not address the fact that the Talmud describes one of the kidneys as providing harmful counsel.)

R. Slifkin continues with a summary of more modern authorities. Finally, he concludes, citing Rambam, R. Hirsch, R. Kook and others, that the best approach is not to view the Bible as a science book, but as a book that speaks the language of men in its times, addressing itself to what people thought in that time. See, for example, Rambam’s explanation of the Bible’s anthropomorphism in his Guide, ch. 26.

The term for the kidneys in Tanach is always based on the word כליות, i.e., it is always in the plural. Scholars theorize that כליה was the singular.

With regard to its etymology, there is no clear answer. Is it related to כל=“all” (symbolizing some kind of multipurpose role of the כליה?) Is it related to כלי =vessel? (This word probably comes from the root כול with its meaning “contain, measure.”) Theological Dictionary of the Old Testament suggests it comes from כל, which it suggests was an onomatopoetic root that refers to rolling and roundness. R. Hirsch (comm. to Lev. 3:4 and Ex. 23:18) derives it from the root כלה and its meaning of longing for something. He writes that the kidneys “represent the innermost source of sensuous desires.” Ibn Ezra (comm. to Ps. 139:13) had also suggested this etymology.

The “long for” meaning of כלה is an expansion from the original meaning: “complete, end.” When you long for something you are exhausting yourself and being completely spent. See, e.g., Ps. 84:3: “nichsefah ve-gam kaltah nafshi.”

I would like to thank Paul Lustiger for asking me about “bochen kelayot,” which got me interested in this topic, and Daniel Klein for pointing me to the article by Rabbi Slifkin.

Mitchell First can be reached at [email protected]. Consistent with the view that the brain is not mentioned in Tanach, when one plans to do something, the typical expression in Tanach is: He said “be-libo,” not “He said in his head,” as we might say today! (The brain is placid, unlike the heart. It is understandable that its importance was overlooked in earliest times.)