How does this work? If it's not literal, how do you know so, and what does it mean? If it is literal, then they relied on the science of their time?
Simcha, Baltimore, USA
(a) There is no doubt that the ancients attributed functions to the parts of the body which today's scientific opinion attributes entirely to the brain. Sometimes, this has important Halachic ramifications. For example, today death is usually defined by cessation of brain activity (or in the vernacular, "brain death"). Halachically, however, one may have to take into account cessation of breathing etc.
(b) In Sefer Yetzirah, one of our earlier compilations attributed to Avraham Avinu, each month of the calendar is assigned a letter of the alphabet, a sign of the zodiac, one of the 12 tribes, one of the senses, and a controlling body part. Thus the month of Nisan is assigned the letter "Heh," the lamb of the Zodiac, and the right hand. It is allocated to Yehudah among the 12 tribes, and to the sense of speech. The month of Av is assigned the letter "Tet," the lion (Leo) of the Zodiac, and the right kidney. It is allocated to Shimon and to the sense of hearing. Elul is assigned the letter "Yud," Virgo of the Zodiac and the left kidney. (According to the reading of the Vilna Ga'on, the right kidney is assigned to the month of Iyar and the left kidney to Av .)
There is obviously a deeper spiritual message in all of this. The different senses are associated with their appropriate months, tribes, letters, zodiacal signs and limbs of the body in order to present an integrated message.
(c) In the Midrash Rabah (Chayei Sarah 61:1) Rabbi Shimon asks, if Avraham Avinu had no spiritual mentor, from where did he learn the Torah? He answers that Hash-m provided him with his two kidneys, which, like a pair of Rabanim, taught him Torah and wisdom. He bases this on the verse in Tehillim (16:7), "I bless Hash-m Who gave me good advice, also at night my kidneys would instruct me."
Similarly, in Midrash Rabah (Vayigash 95:3) we find, "From where did Avraham learn the Torah? Rabbi Shimon said that his two kidneys were like two jugs of water spouting forth Torah."
Sefer Yefeh To'ar asks why specifically the kidneys were mentioned in Tehillim (rather than the Lev or the Sechel). He answers that although the heart understands, clarification of the truth is attributed to the kidneys. This is presumably what is meant by the dictum in our Gemara, "Lev Mevin u'Kelayos Yo'atzos."
If so, with regard to your question we may suggest that kidneys are related to counsel because they perform a cleansing function in the excretory system. By metaphoric personification, they advise and clarify by separating the unfit from the fit.
(d) There is no mention of the brain in the Tanach. It is always the heart, as in, "You shall love Hash-m with all your heart." The kidneys are nearly always mentioned in conjunction with the heart.
It is of course possible that the Torah was given in the language of the times. In those days scientific knowledge was far more limited and the function of the brain was not appreciated. Accordingly, in ancient civilizations its activities were attributed to different parts of the body.
However, I prefer to believe that the Torah does so in order to give a spiritual dimension to the physical accumulation of knowledge and brain power. The "Lev," or heart, is mentioned throughout the Tanach instead of the brain, in order to impart an emotional and central dimension to the human mental faculty. This limits the scope of man's arrogance and self importance which is implicit in thinking that one's brain, or "self," is in complete control. Instead, we are told that each part of the human anatomy contributes to man's cognitive function. This enables us to realize the concept of "Kol Atzmotai Tomarnah Hash-m Mi Chamochah"
Regarding "kloyos yo'atzos", I heard an explanation from Rav Moshe Kupetz of Manchester, that the adrenal glands on top of the kidneys inject adrenalin into the bloodstream when a person is in a situation where he must make an
import urgent decision ("fight-or-flee")
Meir Eliezer Bergman
Here is something that another Meishiv of the Kollel wrote with regards to a question we received a few years ago: (See also what a reader posted on our site at www.dafyomi.co.il/berachos/discuss/bera-061.4c3.pdf .)
The kidneys are said to advise due to the fact that there are two of them, as opposed to other internal organs of which there is just one of each. We consider the right kidney as counseling good and the left one as counseling evil (Maharsha - some Poskim have advised people who are due to have a kidney removed to elect to remove the left one!)
Perhaps Chazal attributed this characteristic to the kidneys due to the fact that their function is to filter the waste from the blood. Blood represents a person's desires, as the Pasuk calls blood Nefesh (Devarim 12:23) and Nefesh is the source of desire (ibid. 12:20). Since the kidneys determine which particles should be removed from the blood and which should remain, Chazal viewed them as advisors.(See also Michtav mei'Eliyahu vol.5 and Nefutzos Yehudah quoted in Ein Yakov.)