Until this point the Alter Rebbe has explained the Talmudic teaching that the World to Come was created by the letter yud: the Sefirah of Chochmah, which this letter represents, is the spiritual source of Gan Eden which is in the World to Come.
However, according to the above, since the letters of the Supernal creative speech derive from Chochmah, it would seem that this world, which was also created by the letters of Divine speech, was likewise created from the letter yud that represents Chochmah. Why, then, do our Sages teach that only the World to Come was created from the letter yud, while this world was created from the letter hei?
The Alter Rebbe therefore now goes on to explain that the letters of speech are composed of "matter" and "form" — the "body" and "soul" (or: the external and internal aspects) of the letters. While the "form" of the letters does indeed derive from Chochmah, the "matter" of the letters of speech merely derives from the heart.
So, too, Above: the internal aspect of the letters derives from Chochmah; from this aspect are created the higher spiritual beings which are able to apprehend G‑dliness. However, the external aspect of the Supernal letters of speech derives from the "breath" — the hei of the Divine Name. It is from this lesser level that this physical world was created.
In the words of the Alter Rebbe:
אך האותיות הן בבחינת חומר וצורה, הנקרא פנימית וחיצונית
However, the letters exist on planes of "matter" and "form", which are also referred to as their "internal" aspect (i.e., the "form", which is their "soul") and their "external" aspect (i.e., the "matter", which is their "body").
כי הגם שמקורן הוא מקדמות השכל ורצון הנפש
Although their source is the primordial stage of the intellect and the will of the soul, for as soon as the soul desires to speak these letters are formed, as explained above,
זו היא בחינת צורת שינוי המבטא שבכ״ב אותיות
this is but the "form" of the differentiation in the pronunciation of the twenty-two letters.
אבל בחינת החומר וגוף התהוותן, והוא בחינת חיצוניותן, הוא ההבל היוצא מהלב
The "matter" and "body" of their formation, however, i.e., the aspect of their "externality", is the breath issuing from the heart.
שממנו מתהוה קול פשוט היוצא מהגרון
From this breath is formed a simple sound which proceeds from the throat,
ואחר כך נחלק לכ״ב הברות וביטוי כ״ב אותיות
and which is then divided into the twenty-two kinds of enunciation and expression of the twenty-two letters,
בה׳ מוצאות הידועות: אחה״ע מהגרון
[these divisions taking place] through the five known organs of speech: alef, chet, hei and ayin1 through the throat;
גיכ״ק מהחיך כו׳
gimmel, yud, kaf and kuf through the palate, and so on; e.g., beit, vav, mem and pei from the lips, and so on;
ומבטא ההבל הוא אות ה׳, אתא קלילא כו׳
while the breath itself, which has its own sound independent of the letter being spoken, is uttered by the letter hei2 — "the light letter," inasmuch as it lacks the substance of a complete letter,
והוא מקור החומר וגוף האותיות טרם התחלקותן לכ״ב
which is the source of the "matter" and "body" of the letters before their division into twenty-two.
The hei is thus the source of each letter's "body".
ולכן אמרו רז״ל שעולם הזה נברא בה׳
And that is why our Sages, of blessed memory, said that "this world was created by the hei": it was created by the external aspect, or "body", of the Supernal letters whose source is the hei of the Divine Name.
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