Baal HaSulam, Shamati #19, “What is the Creator Hates the Bodies in the Work”: We must know that “God hath made even the one as well as the other.” It means that as much as there is disclosure in Kedusha, to that extent the Sitra Achra (Other Side) awakens. In other words, when one claims, “it is all mine,” meaning the entire body belongs to Kedusha, the Sitra Achra, too, argues against him that the whole body should serve the Sitra Achra…
One must know that it is a sign that this work hits the target.
All of nature, all the levels from the bottom up, still, vegetative, animate, and speaking nature, and also every level width wise, every cell, every species, are all based on the connection between opposites. A difference means oppositeness, a plus and a minus. Everything, from the tiniest particles to the bigger structures all connect since they are opposite from one another and can thus add or complement one another.
So nothing can be stable unless it is made of two opposite parts. Only when two opposites can mutually complement one another, despite their oppositeness, can they exist. This is the only way the connection between them can exist and the only way they can exist.
This principle stems from the initial development of the Light, which means “something from something” that created the matter of the desire to receive as “something from nothing.” We can discover the Light only in the desire and the desire can exist only if the Light sustains and revives it.
So in all of creation, every discernment in any thought, in any desire, in any action always includes two opposite parts that are in relative balance. So if there is goodness, there must also be evil in it which balances the goodness. It’s because of the evil that a person can discover the goodness and vice versa. There cannot be one without the other, just as we cannot be balanced standing on one leg, but we always need two legs. So it’s clear that we develop by two opposite forces, and each time one overcomes the other alternately; the right leg is put forward and then the left leg, and this is how one advances.
So when we are not very developed, we think that everything has to be ideal and that ideal means one thing for us. But when we grow, we understand from our experience and by the expansion of our perception that the states must be complex, and that they must always include two forces, two opposite attributes. Only then can they exist, as it says: “and there was evening and there was morning, one day,” only according to this principle.
From the Preparation to the Daily Kabbalah Lesson 12/27/13