It’s a question that lies at the heart of much Kabbalistic thought. And it all starts with light and vessels.
Kabbalah teaches us that light, or Ohr in Hebrew, can't exist in the various worlds without a Kli, a vessel. Think of it like this: you can’t pour water without a cup. The Petichah leChokhmat haKabbalah, the “Introduction to the Wisdom of Kabbalah," explains this concept of spiritual vessels in detail (sections 3 and 4).
Initially, there was just one vessel within the ten sefirot, the emanations of God: Malkhut. When we talk about the five levels – Keter, Ḥokhma, Bina, Tiferet, and Malkhut – these are all, in a sense, parts of Malkhut. It’s called the fourth level because these sefirot are defined by their role in completing the vessel, which is Malkhut (as explained in Petichah leChokhmat haKabbalah, section 5).
But here's where things get interesting. After the tzimtzum, the initial constriction or withdrawal of God’s light to create space for the world, a kind of partition was placed upon the vessel of Malkhut. This partition prevents the supernal light from simply filling it. Imagine a dam holding back a vast reservoir.
So, what happens when the supernal light, that intense and divine radiance, reaches this partition? It collides! It’s a fusion through collision, as the light pushes against the barrier upon Malkhut.
And what happens to the light that’s rebuffed? Well, it doesn't just disappear. Instead, it's reflected back, and this reflected light is called the "ten sefirot of the returning light," or Ohr Chozer. It ascends from below to above and, in doing so, it clothes or encompasses the ten sefirot that are in the supernal light – which are known as the “ten sefirot of the direct light,” or Ohr Yashar.
From this returning light, new vessels are formed! These new vessels are designed to hold the supernal light, taking the place of Malkhut, which, after the tzimtzum, couldn't receive the light directly. The nature of these new vessels, these "ten sefirot of the returning light," is explored in greater detail in sections 14–26 of the Petichah leChokhmat haKabbalah.
So, we see that even in apparent limitation, in the constriction and the partition, there's a dynamism, a constant interplay of light and vessel, of giving and receiving. It's a dance that allows for creation, for relationship, and ultimately, for us to participate in the unfolding of the divine. It begs the question: what partitions do we create in our own lives, and how can we transform them into opportunities for the returning light to illuminate new vessels within us?