The line is also referred to as a “pipe (line)”, meaning to say that it is three- rather than two-dimensional and has content. Here’s how Ramchal explains it in Petach 29:
The Sephirot are what came forth from the mystical space that occurred at the time of the Tzimtzum as we pointed out. But there’s another element here that can only be explained by the three-dimensionality of the “line” — the fact that within each Sephira lies an inner core that came about by the inner light of the line of the Ein Sof entering it.
As Ramchal explained in his comments here, “when the line entered into the trace, a brilliant light shone from the line … which is the soul of each (Sephira’s) vessel”. Thus each Sephira serves as “a garb (i.e., an encasement) for the line”. As such, “the line shines within the inner core of each Sephira with the mystical implications of a soul to the Sephira’s “body”. There’s thus a dynamic ebb and flow between line and Sephira.
(c) 2011 Rabbi Yaakov Feldman
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