Question: What is the upper root of the black letters? From where does their force derive?
Answer: From the tip of the Yod (the 10th letter in the Hebrew alphabet). The Ari wrote in a poem:
Behold that before the emanations were emanated and the creatures were created,
The Upper Simple Light had filled the whole existence.
And there was no vacancy, such as an empty air, a hollow,
But all was filled with that Simple, Boundless Light.
And there was no such part as head, or end,
But everything was One, Simple Light, balanced evenly and equally,
And it was called “the Light of Ein Sof (Infinity).”
And when upon His simple will, came the desire to create the worlds and emanate the emanations….
And then the desire to receive was revealed, something that was opposed to the Light.
The Light is bestowal and love. And suddenly, within bestowal and love something black begins to appear to differentiate from bestowal and love. Actually, it is a small drop, something that isn’t similar to Light, something distinct from it, that was created in it, and it precisely is called “creature.” Everything else developed from it. All the rest was consequently determined by this tiny difference from the Light, which began to develop due to the Light, and it began to feel itself and become familiar with its oppositeness to the Light.
And then we speak about the fourth level in the phases of Direct Light, about Ein Sof (infinity), the restrictions and the continuation of the development of the creature. But actually nothing changes: There remains that same small difference from the Light. The only thing that grows in it is the feeling to what extent it is opposite the Light. But by itself, it is just slightly different from the Light.
Therefore, the Creator only created the tip of the Yod, and all the rest developed from this point. This is the root of the letters.
From the 2nd part of the Daily Kabbalah Lesson 2/1/12, The Zohar