Based on the fact that the soul clothes itself within the body [see Anatomy of Creation], we can understand the meaning of another enigmatically worded verse, "If the blood of the man shall be spilled by a man, his [the second man's] blood shall be spilled, for in the image of G‑d He made man." (Gen. 9:6)
The Ari re-reads the first half of this verse to refer to the soul and the body:
"If the blood of the man…by a man" refers to the blood [ i.e. Nefesh)] of The Man [the Superior Man - Adam Tachton] by a man [which inhabits the Inferior Man - Adam Elyon]. For it is known that the blood is the Nefesh.
In the Torah, the reason given for the prohibition against eating/drinking the blood of any animal is, "For the soul of all flesh is [in] its blood". (Lev. 17:14) In another place, it is written, "For the blood is the soul." (Deut. 12:23) In every case connected with blood, the word that is used for "soul" is by a man.
The same applies to man. Man's Nefesh is said either to be his blood, or in his blood. One way or the other, the Nefesh is directly associated with the blood. Nefesh, as known, is the lowest aspect of the five levels of the soul; the aspect that "comes to rest" (in Hebrew, "nafash") in the body. Nefesh is thus the link between all the higher aspects of the soul, and the body. Man…is a combination of two completely diverse and dissimilar elements, namely, the body and soul…
In Derech Hashem (3:1:1), the Ramchal writes:
Man is different from any other creature. He is a combination of two completely diverse and dissimilar elements, namely, the body and soul [Neshama]. [When we speak of man's soul, however, we are actually speaking of two different things.] One type of soul [Nefesh] that man has is the same that exists in all living creatures. It is this [animal] soul that is responsible for man's natural feelings and intelligence…
In addition, there exists in man a spiritual entity that is very different and much higher [than this animal soul]. The only reason that this entity becomes part of man is to bind him to the Highest Roots… It is through this spiritual entity that the influence bestowed upon man from the Highest Sources is transmitted. From the divine soul, this influence is transmitted to the animal soul, and then to the body.
The Nefesh Elyona [supernal divine soul] directs the Nefesh Tachtona [the lower, animal soul] and through it, performs its necessary functions… The divine soul is bound to the animal soul, which in turn, is linked to the most ethereal aspect of the blood. In this manner, the body and the two souls are bound together [in a chain].
Rabbi Aryeh Kaplan comments on the statement, "The divine soul is bound to the animal soul, which in turn, is linked to the most ethereal aspect of the blood." He writes: When one man kills another, the main damage done is to the second man's body…
Rabbeinu Bachya on Leviticus 17:11 writes that this refers to the last revi'it of blood in the body [a revi'it is a quarter of a biblical lug, i.e. approximately 3 oz.]. In Sota 5a, s.v. Adam, Rashi writes that this is the minimal amount of blood with which a person can live, while Tosafot writes that this is the amount of blood in the heart. A third opinion is that of Rambam on Mishnayot Ohalot 2:2; he writes that this is the amount of blood with which a person is born.
The most significant explanation is that of the Ari in Etz Chaim 42:1. He explains there that this "revi'it of blood" refers to the highest element of the blood, namely, the "essence life-force of the brain", which, in effect, is the interface between the spiritual [divine soul] and the physical [body].
We can understand this on the basis of other statements found in Etz Chaim. The nerves, as well as the veins and arteries, are said to contain the most refined fraction of the "blood". (Etz Chaim 20:5, 40:12, 41:1) The only thing that flows through the nerves, however, is the neural impulses, and therefore these impulses must be considered the most refined fraction of the "blood". This, of course, is the "essence life-force of the brain" mentioned above, since all mental activity depends on neurological impulses. According to this, the "animal soul"… depends on this "blood," namely, the neurological processes. This is the meaning of the statement, "the soul is [in] the blood." The reason for the prohibition against eating all blood would then also be because this neurological activity directly depends on the blood for sustenance. (Aryeh Kaplan, notes to Derech Hashem, Part Three, note 3, p. 347)
Rabbi Chaim Vital continues:
The verse informs us that, when one man kills another, the main damage done is to the second man's body. The reason for this is now given, "For in the Tzelem Elokim [image of G‑d] He made man.?" Unlike Gen. 1:26, where Tzelem and Demut are both mentioned, the verse mentions Tzelem ["image", which is associated with the soul] but not Demut ["likeness", which is associated with the body]. This comes to tell us that the main punishment for manslaughter is for the evil and damage that is inflicted on the victim's soul.
[But this itself could lead to a mistaken understanding.] A man-slaughterer might mistakenly think that, by killing his victim's physical body, he is thereby doing a favor for his soul. After all, he is releasing it from its dungeon! The soul…yearns constantly to rise up [and return] to her Father's house…
For, indeed, the soul is a "portion of G‑d from on high". (Job 31:2) She therefore yearns constantly to rise up [and return] to her Father's house. [Until then] she is dejected, for she is forced to inhabit a material body. If so, the man-slaughterer concludes, he should be rewarded [for releasing her], not punished!
But the Torah has already provided an answer to this distorted view of reality. "For in the image [Tzelem] of G‑d"; this is [again] none other than the spiritual aspect of man, his soul, i.e. "He made man" - the physical aspect, the Asiya-body. For, as we explained, when the soul descends, it dresses up in the atmosphere of this world [which is none other than the body of Asiya].
This makes harming the body tantamount to harming the soul.
[Now, it is known that the descent of] the Shechinah into this lower dimension is for a very exalted purpose [i.e. for the eventual elevation of all the worlds back into divinity]. [As such, it is the divine parallel of the soul's descent into the body.] For the Shechinah is none other than the G‑dly Presence that wishes to [reveal itself and] dwell in all its glory in our physical world.
This last phrase is borrowed from the second half of the verse, "His deliverance is surely near for those who fear Him; the time is close when He will cause the radiant glory of His indwelling presence to be revealed in our land". (Psalms 85:10)
On the other hand, it is known that one who worships idolatry causes the Shechinah to depart from Israel, leaving this world like a dead body [without a soul]. Causing a man's soul to depart and to leave this world is tantamount to causing the Shechinah to depart from Israel…
The man-slaughterer's sin is exactly the same. Causing a man's soul to depart and to leave this world is tantamount to causing the Shechinah to depart from Israel. Why? "For in the image of G‑d, He made man." Thus, just as one who causes the Shechina to depart is deserving of punishment, so also, is one who has caused the soul of a man (which is a portion of G‑d on high) to depart is deserving of punishment.
I have found two sources for the above parallels. First, Yevamot 63b:
We have a tradition: If one does not [marry and thus] try to fulfill the command to "be fruitful and multiply", (Gen. 1:28, 9:7) it is as if he has spilled blood [i.e. committed manslaughter]. It is thus written, "If the blood of a man shall be spilled by a man, his [the second man's] blood shall be spilled, for in the image of G‑d, He made man". (ibid. 9:6) Immediately following this, it is written, "You shall therefore be fruitful and multiply…" (ibid. 9:7)
Rabbi Yaacov explained: It is as if he diminished [G‑d's] presence [in the world], as it is written, "For in the image of G‑d, He made man," which is immediately followed by, "You shall therefore be fruitful and multiply" [i.e. not only to replace one human soul for another, but to replenish the quantum of G‑dliness in the world that was depleted by the loss of human life]… When Cain killed Abel, the Shechinah departed and went up to the 2nd firmament…
Our rabbis have taught us: It is written, "When it [the Ark] came to rest, he [Moses] said, 'Return, G‑d, [to rest Your presence on] the ten thousands of Israel's thousands." (Num. 10:36) We learn from here that the Shechinah does not rest on less than a minimum of two ten-thousands (20,000) plus two thousands (2,000) (= 22,000). If, therefore, there were 21,999 Jews, and such a person desisted from having children, hasn't he thereby caused the Shechinah to depart from Israel!
Abba Chanan said in the name of Rabbi Eliezer: It is written [concerning Nadav and Abihu], "They had no children." (Num. 3:4) Behold, if they had had children, they wouldn't have died!
Others say: [Where do we learn that one who doesn't have children] causes the Shechinah to depart from Israel? It is written [concerning Abraham], "I [G‑d] will uphold My covenant between Me and between you and your descendants after you throughout their generations, an eternal covenant. I will be G‑d to you and to your offspring after you." (Gen. 17:7). When you have offspring, the Shechinah rests [on you] When you have no offspring, upon whom shall the Shechinah rest? On trees and rocks?!
The main source for the concept of departure of the Divine Presence from the world is found in Bereishit Rabba 19:6. There, it is recorded that, when Adam sinned, the Shechinah went up and departed from the earth plane to the 1st Firmament. This means that the consciousness of G‑dliness became occluded and hidden from men's minds. In the generations that followed, the Shechinah made six additional major "departures." The Midrash enumerates all seven stages of this occlusion process:
Rabbi Abba bar Cahana said: It is written, "They heard G‑d's voice moving about in the Garden." (Gen. 3:8) The word [translated as] "moving about" is "mit'halech", a reflexive form of the verb "to walk", indicating that the Shechinah, which was meant to dwell below, was now departing and going up in leaps and bounds. Once Adam had sinned, the Shechinah departed to the 1st firmament. When Cain killed Abel, the Shechinah departed and went up to the 2nd firmament. When men began to worship idolatry during the generation of Enosh, the Shechinah departed to the 3rd Firmament. The sins of the generation of the Flood caused the Shechinah to depart to the 4th Firmament. The sins of the generation of the Tower of Babel caused the Shechinah to depart to the 5th Firmament. The sins of the Sodomites caused the Shechinah to depart to the 6th Firmament. The sins of the Egyptians in the days of Abraham caused the Shechinah to depart to the 7th Firmament.
[Translation and commentary by Avraham Sutton; Likutei Torah (Chumash HaAri, Bereishit, p. 6)]