Behold [the expression] “the Tree of Life” refers to P’nimiyus HaTorah, the inner dimension of the Torah, as stated in [the passage from] the Raya Mehemna, Parshas Naso, [cited above]: “Since in the future, the Jews will taste the Tree of Life which is the Sefer HaZohar.Tanya, Iggeres HaKodesh, 1 explains that the term “the Tree of Life” refers to “the inner dimensions of the Torah and its mitzvos.

To explain: The Written Law is referred to as “the Tree of Life” because it does not enclothe itself in material garments to the same degree [as does the Oral Law]. (For [the Written Law emanates] from Za’er Anpin.) And in [the Written Law], G‑dly light can be sensed as explained in ch. 11.

Similarly, P’nimiyus HaTorah has not enclothed itself in material garments, for instead, it speaks of spiritual matters including the chainlike progression of spiritual existence and G‑dly subjects. Moreover, the conceptualization and the comprehension of these matters is spiritual and in [this type of thought], the G‑dly light can be felt. Its entire intent is to know G‑d and to come to love and fear Him, as the Shaloh writes in his Masechta Shavuos, p. 183b, 2 with regard to the study of Torah lishmah:

The words of Torah that involve research, knowledge, and comprehension [of G‑d Himself] should be studied in order to know His name and His greatness, and the hidden secrets of His mitzvos. Then the person’s heart will be roused to fear Him and to love Him.

This is reflected in the blessing Ahavah Rabbah3 which states: “Enlighten our eyes in Your Torah, cause our hearts to cleave to Your mitzvos, and unite our hearts to love and fear Your name.”

The above is particularly [true in the light of] the teachings of the Baal Shem Tov which were endowed to us as an inheritance by our fathers, the holy Rebbeim, who revealed and explained the greatness and the magnificence of G‑d and all the dimensions of G‑dliness which lead to the love and fear [of G‑d]. [They] taught us to know G‑d and how to serve Him with love and with fear as is fitting, and [showed us] the paths which lead to this, as explained in Tanya, Iggeres HaKodesh, Kuntres Acharon, the passage entitled Lehavin MasheKasuv B’Etz HaChayim:

The knowledge of the existence of the chainlike progression of spiritual existence is also a great and elevated mitzvah. On the contrary, it surpasses all of them, as it is written: 4 “And you shall know today....,” and “Know the G‑d of your ancestors.” 5 And this leads to [service with] a perfect heart.5

See also what is written with regard to this matter in Likkutei Torah, in the explanation of the maamar entitled Lo Tashbis. Note the statements there in chs. 4 and 5.

{I also found a genuine manuscript (- I am not sure whether it is from the Mitteler Rebbe {- this is most likely -} or the Tzemach Tzedek6)which says:

“The statements of G‑d are flawless; He is a shield to all who take refuge in Him,” 7 those who bring them out will find life; 8 i.e., true life, eternal life, he will see [his portion of] the World to Come in his lifetime, 9 and delight in G‑d, 10 in the service within the heart, i.e., in prayer, and in true deeds of kindness.}

Thus it is P’nimiyus HaTorah which is the Tree of Life, which is the revelation of G‑dliness, which leads to “a perfect heart,” i.e., the love and fear [of G‑d] which is the essence of our occupation with the Torah and its mitzvos.

For this reason, the essence of P’nimiyus HaTorah was revealed in these later generations. Thus Tanya, Iggeres HaKodesh, Epistle 26, quotes the AriZal as stating that it is in these later generations that it is permitted - and indeed, it is a mitzvah - to reveal this wisdom. For in the earlier generations, this was not necessary. They were totally righteous men [whose souls] stemmed from high [spiritual] rungs. Because of the tremendous power of their souls, they possessed genuine love and fear [of G‑d] and studied the Torah lishmah. [To attain these levels,] they did not require the revelation of P’nimiyus HaTorah. [Therefore these teachings were hidden, for] “It is the glory of G‑d to conceal a matter.” 11 In [these] later generations, by contrast, the souls do not stem from such high rungs, and [the spiritual potential of our] hearts has diminished. Therefore, it is a mitzvah to reveal [this wisdom].

In each successive generation, the revelation of P’nimiyus HaTorah becomes even more necessary so that [it becomes possible] to know G‑d and [experience] love and fear of Him. Since the spiritual level of the souls have descended and they have become more materially oriented, and there are a multitude of veils and concealment, [were it not for the revelation of P’nimiyus HaTorah,] even those who cling to the Torah would not know G‑d, and would not [experience] genuine love and fear [of Him].

For love and fear are dependent on the knowledge and comprehension of G‑dliness. How is it possible to love something when you do not know what it is? Similar concepts apply with regard to the fear [of G‑d] as explained in Reishis Chochmah, Shaar HaYirah, ch. 1. As explained in Tanya, ch. 3, love and fear are generated from the intellectual attributes, Chochmah, Binah, and Daas. For this reason, these intellectual attributes are called mothers and the source for the emotions.

In ch. 12 it was explained that without the love and fear [of G‑d], the study of the Torah could also become a potion of death. Therefore, in G‑d’s kindness to us, He revealed to us the Zohar which had been hidden since the death of Rabbi Shimon so that it would be revealed in the later generations, as it is written in the Tikkunei Zohar: “And some people will sustain themselves on the lower plane from your composition [the Zohar] in the later generations, in the end of days.” And in each subsequent generation, in His kindness, G‑d has revealed the light of P’nimiyus HaTorah through His faithful servants.

Accordingly, it is a mitzvah and an obligation for each and every one to study P’nimiyus HaTorah. The fundamental aspect of our Torah study should be to know what is forbidden and what is permitted and the laws of ritual impurity and purity in order to refine the world as stated in Tanya, Iggeres HaKodesh, Epistle 26, and in Kuntres Acharon, the passage entitled Lehavin MasheKasuv B’Etz Chayim. We must amplify our study and observance of all the 613 mitzvos in actual practice in thought, speech, and deed, for they relate to the worlds of Beriah, Yetzirah, and Asiyah, facilitating the task of refinement carried out there. Nevertheless, there is a duty which is both a mitzvah and an obligation: to study P’nimiyus HaTorah as stated in the source in Iggeres HaKodesh cited above: “It is a great and lofty mitzvah, which indeed surpasses all of them.”

A person who does not study P’nimiyus HaTorah is liable for his soul, for even if he occupies himself with Torah study, he will fall from his spiritual level as explained above.

Not only will he not carry out the task of refinement, on the contrary, he will.... 12

Certainly, this applies to someone who does not desire this study, but rather [to devote himself] solely to the study of Nigleh, the revealed, legal dimension of the Torah. He attaches himself to the place of death as stated in the passage from the Zohar, Parshas Vayechi, cited above.

For it is through the study of [P’nimiyus HaTorah] alone that a person will merit the light of life. [Indeed,] it is “the Tree of Life,” and the medium which will bring a person to the love and fear [of G‑d]. And [then], he will “live in them,” 13 i.e., his Torah study will enable him to carry out the task of refinement and elevate his soul - and even the source of his soul - and unite them in His oneness. This is the intent of our Sages’ statement: 14 “All of these [forms of disease] can be nullified by bread [dipped] in salt.” Salt, an analogy for the study of P’nimiyus HaTorah, is necessary, as stated in ch. 4.

Synopsis: [This chapter] explains that the “Tree of Life,” the study of P’nimiyus HaTorah, is not enclothed in material garments; its intent is to attain the knowledge of G‑d and to reach the love and fear [of Him].

[P’nimiyus HaTorah] is revealed essentially in the later generations when the souls are on lower levels, and the concealment [of G‑dliness] has increased.

Whoever does not study [P’nimiyus HaTorah] is liable for his soul. Certainly this applies with regard to a person who rejects this study.