Nefesh HaSichlit—The Intellectual Soul
The Nefesh HaSichlit is the intellectual soul. It is an innovation of the Chasidic philosophy of Chabad.  Rabbi Schneur Zalman of Liadi, the Alter Rebbe, said he had not learned of this (Nefesh HaSichlit) from the earlier sources but rather reached this understanding on his own. In his own words, “G‑d enlightened my eyes.” 
The Tanya speaks of two souls— Nefesh HaBahamit and Nefesh HaElokit because these two are the essential players in the inner conflict of the human soul.  However, in another place, the Alter Rebbe speaks of three souls—Nefesh HaBahamit, Nefesh HaElokit, and Nefesh HaSichlit.  According to Rabbi Menachem Mendel of Lubavitch, the Tzemach Tzedek: “Nefesh HaSichlit is part of Nefesh HaBahamit.”  However, according to Rabbi Sholom Dovber Schneersohn, the Rebbe Rashab, “Even though Tanya describes two Nefashot [souls], there are really three—Nefesh HaElokit, Nefesh HaSichlit, and Nefesh HaChiyunit.”  Nefesh HaChiyunit is used here, apparently, as synonymous with Nefesh HaBahamit.
The big picture that emerges from various Chabad sources is this: generally, there are two souls—Nefesh HaElokit, the godly soul, and Nefesh HaBahamit, the animal soul. However, a closer examination reveals a third soul, Nefesh HaSichlit, the intellectual soul. Whereas the godly soul and the animal soul pull the body in opposite directions, the intellectual soul, being in the “middle,” decides between the two. It is the intellectual soul that gives us free choice.
The Alter Rebbe has explained this concept at length in Likutei Torah.  The purpose of the descent of Nefesh HaElokit into the human body is the rectification of Nefesh HaBahamit. However, as mentioned before, Nefesh HaBahamit is self-centered, whereas Nefesh HaElokit is G‑d-centered. As such, they are opposites and have no nexus. How then can Nefesh HaElokit and Nefesh HaBahamit affect each other? Nefesh HaSichlit (the intellectual soul) serves as the connector between the two:
Nefesh HaElokit (the godly soul) can [vest itself] in Nefesh HaBahamit (the animal soul) through…. Nefesh Hasichlit (the Intellectual soul), which is the intermediary between them and connects them…. In order for Nefesh HaElokit (the godly soul) to be able to effect the rectification of Nefesh HaBahamit (animal soul), there is a need for an intermediary, which is the purpose of Nefesh Hasichlit (the intellectual soul). —Rabbi Menachem M. Schneerson 
Thus, the Alter Rebbe states in the Likutei Torah:
The intellectual soul refers to Nefesh HaSichlit, which is an intermediary between a person’s animal soul and his godly soul. 
The need for this soul can be further seen from the metaphor used by the Alter Rebbe in Tanya, where he describes the two souls—Nefesh HaElokit and Nefesh HaBahamit—as two armies fighting for the same city, the person’s body, as mentioned above. The G‑d-centric Nefesh HaElokit pulls the person to serve G‑d and do G‑d’s will, whereas the egocentric Nefesh HaBahamit pulls the person to satisfy his or her own desires.  The question is, who is the arbiter in this tug of war? Who shall decide whether to obey the impulses of the godly soul or those of the animal soul? The answer proposed by the Alter Rebbe is Nefesh HaSichlit, intellectual soul, which serves as a dispassionate arbiter deciding whose advice to follow. 
It appears that, according to the Tzemach Tzedek, Nefesh HaSichlit was given to Adam and Eve when their eyes were open as the result of their partaking of the fruit of the Tree of Knowledge:
…this means [their eyes were opened to] the aspect of external wisdom, the Chokhmah of the Nefesh HaSichlit (Intellectual soul), which is not how it was before [the sin when] they also had wisdom, but it was the Chokhma of the divine [soul]. —Tzemach Tzedek 
Thus, Nefesh HaSichlit is viewed as merely “human intellect relating to physical matters.” [58,59] For this reason, Nefesh HaSichlit is called Adam. Initially, Nefesh HaSichlit is seen as at best mundane or worse, opposed to the divine consciousness: “the intellect which grasps physicality according to the human intellect; and it is falsehood and indeed the opposite of the divine wisdom, which is the wisdom of truth.”  However, the intellectual soul can be rectified and elevated by Torah study. Nefesh HaSichlit thus can be transformed when it
…is elevated and connected with the light of Torah … until the wisdom that is in the Nefesh [HaSichlit] will be … absorbed in the light of the Torah and to be truly transformed from one extreme to the other, from the human intellect, which [merely] grasps [the physical], to the intellect of the wisdom of the Torah – godly wisdom. 
Only after the intellectual soul is refined can it, in turn, refine the animal soul—Nefesh HaBahamit. 
The Nefesh HaSichlit possesses both intellect faculties (sechel) and emotional faculties(midot). 
 This statement is puzzling, however. Aristotle already spoke of an intellectual soul. In Aristotle’s metaphysics, living beings are thought of as a composition of matter and form. The body is matter whereas the soul is form. The body is the first potentiality and the soul is the second potentiality and the first actuality. Thus, Aristotle wrote, “those who say that the soul is a place of forms are right, except that it is the intellectual soul, not the whole soul, which is – potentially, not actually – the forms.” De Anima, III, 429a28-29. Following Aristotle, Maimonides and many medieval Jewish philosophers wrote about the intellectual soul. Similarly, Plato spoke of logos—the rational (level of) the soul. Medieval Jewish philosophers who were Neo-Platonists (such as Salomon ibn Gabirol), also wrote of an intellectual soul. Perhaps, the Alter Rebbe distinguishes his understanding of the Intellectual soul as the arbiter deciding between the godly soul and the Animal soul, from the intellectual soul of classical philosophy.
 Sefer HaSichot ??
 This inner conflict violated Plato’s principle of non-contradiction, according to which it is impossible for a person to desire something and, at the same time, desire its opposite. Thus, Socrates states: “It is obvious that the same thing will never do or suffer opposites in the same respect in relation to the same thing and at the same time” (Republic, Book V, part 5). The essential two-soul dichotomy, wherein each soul pools in the opposite direction, is the unique feature of the Alter Rebbe’s scheme, which sets it apart from the unity of soul in classical philosophy. However, the Alter Rebbe was not the first to break with the principle of non-contradiction. According to Aristotle, Heraclitus denied the law of non-contradiction (Metaphysics, Book 4, section 1005b). As pointed out above, these two souls are essentially two facets of the same soul. However, this hidden essential unity is not felt until the godly soul converts the animal soul and compels it to join in the service of G‑d, which is the hallmark of a tzadik (a righteous and holy person).
 Sefer HaMa’amarim Admur HaZaken 5569, Bi’ur D”H, Mayim Rabim, 44.
 Rabbi Menachem M. of Lubavitch (the Tzemach Tzedek), Kitzurim Vchaoros Shel Admur HaTzemach Tzedek, L’Tanya, p. 88)
 Rabbi Sholom Dovber Schneersohn, Sefer HaMa’amarim RaNaT, Dibur Hamaskil “Lechol Habalah,” Kehot, p. 5659.
 Just as the godly soul possesses both intellect and emotions, so too, the animal soul possesses intellect and emotions. (See Sefer HaKitzurim L’Tanya, p. 81.; the maamar VaYavo Amalek, 5709 (Kuntres 62), ch. 2). As the Rebbe Rayatz, Rabbi Joseph I. Schneersohn writes, “Each of the three Nefashot [souls] has sechel [intellect] and midot [emotions]” (Kubutz “Chatomim,” chovros 3, p. 66).
 Likutei Torah, B’chukotai, 47c; ma’amar “Chaviv Adam”, Shavuot 5728.
 Rabbi Menachem M. Schneerson, Maamar “Chavivin Yisroel,” 2nd Day Shavuot 5726, 2‐3; Likutei Torah, Bechukotai, 47c. This and several other sources have been quoted in the online issue of Inyonei Moshiach and Geulah, “All That Remains Is to ’Open Up the Eyes,’” Teves, 5774, section “The Intellectual Soul (Nefesh Hasichlis),” accessed November 24, 2021, https://moshiachindepth.files.wordpress.com/2013/08/kuntres11_openeyes_notbooklet.pdf.
 Rabbi Schneur Zalman of Liadi, Likutei Torah, Ha’azinu, “Ha’azinu HaShamayim…”
 “G‑d made one opposite the other” (Kohelet 7:14, as explained in Tanya, ch. 6).
 The three souls—the godly soul, animal soul, and intellectual soul—are an example of soul trichotomy. It is somewhat akin the trichotomy of Nous (“mind”), Psychë (“soul”), and Söma (“body”) in Platonic philosophy (Plato, Timaeus, 30). Nefesh HaBahamit parallels Plato’s Söma-body, Nefesh HaElokit parallels Plato’s Psychë-soul, and Nefesh HaSichlit parallels Plato’s Nous-mind. It can also be compared with the Aristotelian soul-trichotomy: Threptike (nutritive, vegetative), Aisthetike (sensitive, animal), and Noetike (rational, human), although this analogy stops at the tripartite division (Aristotle, On the Soul). While Noetike can be compared with the intellectual soul, both Aisthetike and Threptike can be compared with the animal soul. No analogy exists for the divine soul in Aristotelian trichotomy. Plotinus’ trichotomy of the hypostases—the One, the Intellect, and the Soul—is also worth noting (Lloyd Gerson, “Plotinus” ). In modern philosophy Alter Rebbe’s trichotomy may be compared to Hegel’s Subjective Spirit, Objective Spirit, and Absolute Spirit (Redding, Paul (1997, 2006), “Georg Wilhelm Friedrich Hegel” in the Stanford Encyclopedia of Philosophy) or Freud’s id, ego, and super-ego, where the id parallels the animal soul, the ego parallels the intellectual soul, and the super-ego parallels the divine soul, although these parallels are tenuous at best.
 Tzemach Tzedek, Ohr Hatorah, Devarim Hanachot, Hoisafot, p. 110.
 Mitteler Rebbe, Vayikra II, Bechukotai, p.339.
 Note that the identification of the intellectual soul with the human intellect harkens back to the notion of nous (Ancient Greek for intellect) in classical philosophy, which was identified with logos in Plato’s tripartite theory of the soul or with basic understanding and awareness in the philosophy of Aristotle.
 Rabbi Schneur Zalman of Liadi, Torah Ohr, Vayechi, 38b.
 Idem., 38c.
 Alter Rebbe, Sefer HaMa’amarim 5566 I, “Lehavin Shoresh Hadvarim,”p. 417.
 The Rebbe Rayatz wrote: “Each of the three Nefoshos [souls] has sechel [intellect] and midot [emotions]” (Kubutz “Chachomim,” chovros 3, p. 66).
Originally published on QuantumTorah.com on 2021/12/15.