The letters themselves, the אותיות—letters, are made up of two aspects: material and form. In the language of the Alter Rebbe, this is called חומר וצורה—material and form, or as he puts it, פנימית וחיצונית—the inner and the outer.

Just as a body has a physical substance and a soul that gives it life, so too with the letters. The form—the inner aspect, the פנימית—is the source of the letters, which comes from the earlier stages of the intellect and the desire of the soul. This is the neshama of the letters, the spiritual drive, the will to express something.

But then, there is also the material side—the חיצונית, the external. This is the actual physical sound, the way the letters are pronounced and heard. The soul wants to speak, but the words themselves become something tangible, something that exists in the physical world. This is the aspect that the world can relate to, the actual sound and articulation of the letters.

So, even though the source of the letters is rooted in the intellect and the will of the soul, what we actually hear and see as letters is their external, material form. The form is the spiritual intent and meaning, while the material is the physical expression and sound.

Now, the Alter Rebbe explains more about the material side of the letters—their חומר, their body. This is called their חיצוניות—the external aspect. Where does this material aspect come from? It originates from the breath that comes from the heart. This breath creates a simple, undifferentiated sound—the kol poshut, the simple voice—that emerges from the throat.

After this simple sound leaves the throat, it is then divided into the 22 different expressions, the כ"ב הברות—the 22 articulations, which become the 22 letters. This division happens through the five known exits of the mouth, as the Alter Rebbe lists: אחה"ע מהגרון—the letters Alef, Ches, Hei, Ayin come from the throat; גיכ"ק מהחיך—Gimel, Yud, Chof, Kuf from the palate, and so on.

The expression of the breath itself is represented by the letter ה'—the Hei, which is called a "light letter." This is the source of the material and the body of the letters, even before they are divided into the 22 forms. The Hei is the general breath, the undifferentiated sound, before it becomes specific letters.

This is why our sages say that this world was created with the letter Hei—שעוה"ז נברא בה'. The Hei represents the source of the material aspect of all letters, and from it, all the different letters and expressions are formed. The world, in its physicality, comes from this general breath, which is then articulated into specific forms and letters.

So, the process is: first, there is the desire and intellect—the inner form. Then, the breath from the heart creates a simple sound. That sound is divided into the 22 letters through the five exits of the mouth. The letter Hei is the root of this process, representing the material source from which all letters and, ultimately, the world itself are created.