The first act, the act of Tsimtsum, in which God determines, and therefore limits, [Themself], is an act of Din [Gevurah] which reveals the roots of this quality in all that exists; these “roots of divine judgement” subsist in chaotic mixture with the residue of divine light which remained after the original retreat or withdrawal within the primary space of God’s creation. Then a second ray of light out of the essence of En-Sof brings order into chaos and sets the cosmic process in motion, by separating the hidden elements and moulding them into a new form. Throughout this process the two tendencies of perpetual ebb and flow—the Kabbalists speak of hithpashtuth, egression, and histalkuth, regression—continue to act and react upon each other. Just as the human organism exists through the double process of inhaling and exhaling and the one cannot be conceived without the other, so also the whole of Creation constitutes a gigantic process of divine inhalation and exhalation.
Gershom Scholem, Major Trends in Jewish Mysticism; Seventh Lecture: Isaac Luria and his School