Sitra Achra and Tumah

Sitra Achra is a general term for all reality that is not holiness - including permitted matters, while Tumah is a specific term for things that oppose holiness.

Sitra Achra

The Baal HaTanya writes: "Sitra Achra means 'the other side' (that is not the side of holiness), and the side of holiness is nothing but that which is nullified to Hashem."

Purpose of Its Creation

The purpose of creating the Sitra Achra is to enable choice between good and evil. Since Hashem wants man's service, that man himself should choose holiness, it was "necessary" (so to speak) to create a reality that opposes holiness. Thus, Hashem's will includes both the creation of the side of holiness and the creation of the "other side" (the Sitra Achra).

From the External Will

However, there is a great difference between Hashem's will in creating holiness and His will regarding the Kelipos (shells/husks). His will for holiness is "internal" - this is what He "truly" wanted, while His will for the Kelipos is "external" - only for a secondary purpose (to enable choice). Hashem is not interested in the Kelipos for their own sake (and therefore when they complete their role, they will be nullified forever).

The Two General Levels

Kelipat Nogah

The source of all Kelipos is the "fourth Kelipah called Kelipat Nogah of this world, which is called the world of Asiyah".

That is, at the bottom of the world of Asiyah there is a partition that separates between holy and mundane (between the ten Sefirot of Asiyah of holiness and the ten Sefirot of Kelipat Nogah), and through this partition, the aspect of the mundane (Kelipat Nogah) branches off from the holy.

In several places, Kelipat Nogah is called the aspect of Atzilut of the Kelipos. This means that the relationship between it and the three completely impure Kelipos (called Briah, Yetzirah and Asiyah of Kelipah) is similar to the distance in value between the world of Atzilut and the worlds of Briah, Yetzirah and Asiyah.

Intermediary Between Holiness and Kelipah

In Kelipat Nogah, good is still mixed in, which is why it is an "intermediary" between holiness and impurity. This is expressed in that sometimes it rises and is completely included in holiness, and sometimes it is included in the three impure Kelipos. Its ascent to holiness and vice versa depends on man's service: when he uses worldly matters for the sake of holiness, the sparks of holiness within them are refined and rise to holiness, but when he uses them for his personal matters (even if not forbidden), then a. he brings them down to the three impure Kelipos, and b. falls with them, and c. receives his vitality from them (from the three impure Kelipos) - until his body becomes a garment and chariot for them (until he repents).

The Three Impure Kelipos

Below Kelipat Nogah there is another partition, more physical, which separates between Nogah and the three completely impure Kelipos. The three impure Kelipos themselves are the overall vitality of the seventy ministers, from whom all prohibitions, impure animals, and lands of gentiles receive their vitality.