The Zohar, Chapter “Bo (Come Unto Pharaoh),” Item 118: A firstborn is considered Hochma, and “All the firstborn” indicates that even the higher and lower degrees were broken from their dominion… ” Item 119: “From the first-born of Pharaoh that sits upon his throne…even unto the first-born of the maid-servant…."
A firstborn is GAR, the head of the Partzuf, and it needs to subside in Egypt. Everything that was perceived in the desire to receive pleasure as a stable basis of life, which supposedly could be achieved egoistically, is now being destroyed: "From the firstborn of Pharaoh unto the firstborn of the maid servant and the firstborn of the cattle."
Egoism grows, the Light cannot fill it, and Egyptian darkness comes about. It seems to us that the attitude of the Upper Governance with regard to us changes, but in reality the Light exists in absolute rest while the desire to receive pleasure in us increases constantly. If we don’t correct it in time, we feel greater and greater darkness and suffering within it.
At the end of the Egyptian slavery our desire increases through a leap, and it is revealed with a new force on the next level. It is there that a person feels Egyptian darkness, the ten plagues, which is egoism revealed in all of its ten Sefirot, meaning in a complete desire displaying darkness and evil. The desire has grown, and we have stopped feeling even that minor illumination within it which we had experienced before.
The ceasing of the former Light in the new, great desire to receive pleasure is called the execution of all the firstborns in the dark night, "from the firstborn of Pharaoh unto the firstborn of the maid servant and the firstborn of the cattle." This means that the Light has been concealed and is no longer revealed.
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