The Medrash says that the divine name Sh-dai comes from the word dai, “enough,” for G–d said: “I am the One Who said to My world ‘enough,’ and to the heavens and earth ‘enough,’ for had I not said so, they would have continually expanded” (Bereishis Rabba ch. 46, Chagiga 12a, cf. Torah Ohr 56c). Thus, this divine name represents limitation, and its energy created the limitation found in all the spiritual realms.
To explain: All existing things consist of a tzura—a “form,” vested in a chomer—a “substance.” The limitation of each entity stems primarily from its chomer.
For instance, in the heavenly bodies, the tzura consists of the soul of the entity, while the chomer consists of a combination of the elements of fire and water. The same is true of the worlds of Yetzira and Beriya.
The limitation in the worlds of B'ya (Beriya, Yetzira, and Asiyah) stems from the limitation within the Sefiros (divine attributes; sing. Sefirah) in the supernal world of Atzilus, which is the highest of the worlds.
Let us explain these divine attributes, and how they come to be limited:
Every Sefirah in the realm of Atzilus contains an Ohr, a “light” (pl. Oros) and a Keli, a “vessel” (pl. Kelim). The Ohr vests itself in the Keli. We can understand this through understanding the middos, the character traits (sing. midda) within a person, which stem from and parallel the Sefiros.
By nature, every midda extends without limit.
Thus, the nature of the attribute of kindness, Chessed, is to be kind and forgiving even to the wicked. Of this our sages say: “How great is His kindness” (Avos 5:2).
Conversely, the nature of the attribute of strictness, Gevurah, is to be harsh and unforgiving even when approached by a sincere penitent. This can also be explained from the perspective of divine Gevurah: it is written that “In G–d’s eyes even the heavens (i.e., the angels) are unworthy” (Iyov 15:15). This means that from the perspective of His pure strictness no sin can be forgiven, no matter how great one’s remorse.
This tendency to unrestrained expression of the character trait is the aspect of the Ohr within it.
However, the Keli of the midda restricts it and prevents it from being expressed inappropriately. For example, it ensures that one’s quality of kindness does not overstep its boundary by displaying kindness to the wicked; likewise, it ensures that one’s quality of strictness not show harshness to one who truly deserves kindness.
The reason that the Ohr expresses the unlimited, while the Keli limits, stems from the inherent nature of these two forces:
The Ohr is by nature simple and undifferentiated (“poshut”); thus, the Ohr of the midda does not discriminate at all in the way it is revealed, for that would entail a certain limit.
In contrast, the Keli is by nature limited and defined; thus, the Keli of the midda serves the role of limiting the midda’s expression.
To put it differently: Since the Ohr of the midda is simple and unlimited, it can’t tolerate any other midda. On this level, each midda wants only its own expression, to the exclusion of all others.
However, the Ohr of the midda is vested in a Keli. This limits the midda’s expression, thereby enabling it to tolerate other middos, and even complement them.
All the above is also true of the relationship between the Oros and Kelim of the Sefiros of Atzilus. It is these Kelim that act as the source for the limitation in B’ya, as mentioned, and this is the idea of the divine name of Sh-dai.