Ramchal differentiates between the Sephirot and Ein Sof Himself by referring to the Sephirot as “Emanated Light” as opposed to God’s own light which is “Simple” or pure and unchanged. We’ll explain that shortly, but first why use the term “light” when referring to such intangible entities? Wouldn’t it be better to avoid using material terms altogether?
For one thing, we refer to it by a term “only to give it some sort of name”, as he puts it. That’s to say, we simply can’t study God’s workings in this world as we’re expected to without reference-points and names, so we use the term “light”. It’s most appropriate because “light is the finest and most subtle of all physical things” as he points out.
Again, the point is that in order to depict the Sephirot and to contrast them with God’s own being, which is indescribable (as we said in 1:2), we’d need to contrast their light-nature with His. As such, the Sephirot are termed “Emanated Light”, which means to say, light that had been extruded and derived from a light-source which is God’s own, which is itself “Simple”, for lack of a better term .
 For more on “Emanated Light” see Petach 25, Klallei Ma’amar HaChochma 4, Ginzei Ramchal pp. 297, 307, and Eitz Chaim 1:3. For more on “Simple Light” see Ginzei Ramchal p. 297 and Sha’arei Ramchal p. 50.
(c) 2011 Rabbi Yaakov Feldman
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