The Gemara at the beginning of Yevamot (3b-4a) discusses the rule that a Mitzvat Aseh overrides a Mitzvah Lo Ta'aseh in certain circumstances. We learn this rule, according to the Gemara, from the juxtaposition in this week's parsha of the prohibition of Shaatnez immediately followed by the Mitzvah of Tzitzit.

The prohibition of Sha'atnez is part of the larger prohibition in the Torah against Kilaim - a mitxture of non-like animals, crops or materials. Rabbenu Bachaye in Parshat Kedoshim (19:19) discusses the Mitzvah of "Et Chukotai Tishmoru." What does this prelude refer to? The pasuk continues "Behemtecha Lo Tarbia Kilaim, Sadecha Lo Tizra Kilaim Ubeged Kilaim Sha'atnez Lo Ya'ale Alecha." According to Rabbenu Bachaye, this Mitzvah tells us that everything needs to have its place in proportion. The word "Kilaim" comes from the root Kaf-Lamed-Alef, which means to prevent. As in a Beit Kele, is a prison, which prevents the inmates from doing things or going places.

Rabbenu Bachaye goes on to discuss how these laws are not just Mitzvot, they are Chuka - engraved in the world. There is writing that we accomplish through marking a surface, and there is writing that we accomplish through engraving it into something, making the writing part of the medium. These Mitzvot of Kilayim are not just recorded, they are engraved. They are "Chukot" - part of the essence of this world.

The world that we know mirrors the world above. The world was created with many forces, all competing with each other, and each having an opposite. These forces have their roots in the world above, and it is in the harmony of these forces in completing each other that the world can exist peacefully. That is why the words "Shalom" (peace) and "Shalem" (complete) come from the same root.

Each of these heavenly forces is appointed over certain realms and functions in this world, and the events in this world are derived from the heavenly forces. In order for the world to be complete, the forces have to be acting in harmony. Such completeness, such peace, is necessary even above, as we see from the pasuk in Iyov, "Oseh Shalom Bimromav". The peace of the world above is that these forces have the proper influence in the world below. As such, when things are kept in their proper place and in their proper proportion in this world, only then is there completeness and peace in the world above. But when things are mixed improperly, brought together so that they are not in the proper place or proportion, the heavenly forces become intermingled and cancelled out, and are disconnected from their proper functions. Thus, "Kilaim" PREVENTS the heavenly forces from carrying out their proper functions.

There are two instances, however, where Kilaim, particularly the mixture of wool and linen, are not only allowed, but are commanded: Tzitzit and the Avnet worn by the Kohen in the Beit Hamikdash during the Avoda. How is it that this improper mixture becomes not only allowed, but required in these cases?

Tzitzit and the Beit Hamikdash go back to the roots of all of the forces in the world. The Beit Hamikdash connects heaven and earth. The Even Hashtiya, from which the rest of the earth was created, is located in the Beit Hamikdash. Even the Zohar talks about how Yerushalayim, particularly the Beit Hamikdash, represents all of the double and competing forces in the world. The Zohar (Chaye Sara 128:2) relates that Rav Eliezer asked his father, R' Shimon Bar Yochai, about the Me'arat Hamachpela. The Torah refers to the place as both "Me'arat Hamachpela" and "Me'arat Sede Hamachpela." R' Eliezer asked whether it was the cave or the field that was called Machpela. R' Shimon Bar Yochai answered that neither the cave nor the field were called Machpela on their own, but that both derived the name from the original Machpela - the original "doubler" - Yerushalayim. In the Me'arat Hamachpela, there are buried the people who represented the harmony of the double forces - the Torah doubles the names: Avraham, Avraham, and Yaakov, Yaakov. The people buried here are the people who succeeded in connecting with the root of all of existence and bringing that connection into this world through their deeds.

Yerushalayim of this earth mirrors Yerushalayim shel ma'ala. Yerushalayim contains within it all of Eretz Yisrael, and Eretz Yisrael contains within it the entire world. In fact, in the Torah, the name for the city is spelled without the yud - Yerushalem - meaning a complete city, or a city of peace. But we pronounce it Yerushalayim, indicating that it is double, like Yadaim, or Raglaim. Thus we have a reference to the completeness and peace of the world above that exists when things are in their proper proportion, and are matched with their proper opposite. As a reminder of this, the Kohen wears the Avnet, a belt that is made of blue (wool) and red (linen) cords twisted together, while he is doing the Avoda.

Tzitzit is the combination of white and blue, linen and wool. Why blue? Because the blue reminds us of the sea, which reaches out to the sky (at the horizon), which reminds us of the Heavenly Throne, which we are told in the Torah and in the Navi looks like a sapphire, as pure as the heavens. The Hebrew name for the blue thread - techelet - has the same linguistic root as the word "tachlit", meaning end purpose. The similarity in the words indicates that when we look all the way out as far as we can see, the shade that we see at the end of our vision is blue - the same shade as the blue wool.

According to the Zohar (Shlach 175:1), clothes can betray a person. If a person walks into a room very well dressed, he will be treated with respect even if he is the most disreputable person around. But if a great Gadol walks into a room dressed in shabby clothing, he will be treated with disrespect regardless of who he is. R' Natan MiBreslov (Hilchot Tzitzit, Halacha B) says that Tzitzit are meant as a correction for the clothes, by connecting the clothing with the real person who wears them, and bringing the proper level of Kavod straight from Hashem.

The Beit Hamikdash and Tzitzit also accomplish a unification of opposites. In the Beit Hamikdash, the place where the Kohen Gadol did the first tevilah before beginning the Avoda on Yom Kippur - the holiest person at the holiest time on the holiest day of the year, beginning the holiest level of work - was in the "Beit Haparva". According to the Gemara (Yoma 35:1), this place was built by a sorcerer named Parva Amgusha. This person engaged in witchcraft, which the Gemara in Shabbos (75:1) says is worse even than Avoda Zara. Avoda Zara is something we are allowed to learn about so that we can teach against it. Witchcraft is something so evil that we are not even allowed to learn about it at all. Yet the place where the holiest of holy work on the holiest day of the year began is a place that was built by, and named for, a practitioner of witchcraft. This is to show that in performing the Avoda, the Kohen brings ALL of the forces in the world together to form the level of completeness that is necessary to achieve true peace.

With regard to Tzitzit, Hevel brought the sheep, the highest level of animal, known for its delicate nature and lack of ego, while Kain brought flax, the lowest form of vegetation. According to the Zohar (Shlach 175:1), Techelet represents the trait of judgment (din), while white, represents the trait of kindness (chesed). It is when one can distinguish between these two colors, between these two traits, that there is enough light in the world to recite Kriyat Shma in the morning. When these two traits are indistinguishable, there is only darkness.

The Midrash (Bamidbar Raba Chukat 19:1) quotes Iyov (14), "Who makes pure out of the impure, not One?" Avraham was born from Terach, Chizkiyahu was born from Achaz, Yoshiya was born from Amon, Mordechai was born from Shimi, the Jewish nation was born from idol worshippers, and the world to come is born from this world. And just as pure comes from the impure, we have the Para Aduma. Those who deal with it become Tameh, while those it is applied to become Tahor. Hashem provided this Mitzvah as a Chok, from the same root as "Chuka". It is engraved in the world as part of its essence. And the contradiction is meant to remind us that all of the forces, good and evil, tahor and tameh, come together to form the level of completeness that is necessary for peace. At the root, they are all one.

Tzitzit act in the same way. It can save us from the worst behavior because it connects everything together in honor of Hashem. It is therefore permitted to wear Tzitzit in the bathroom. This is the most defiled place in a building. A person cannot bring in Tefillin, which contain kedusha. But Tzitzit, which connects everything together, includes even the most defiled in connecting to the root of ALL forces.

Thus we see that Kilaim is prohibited because it is the improper mixture of forces that comes to prevent the proper course of the world, prevent completeness and prevent peace. But in two instances - the Avnet of the Kohen's Avoda, and the Tzitzit that correct our clothes, we are connected to the root of all forces and the unification of all forces that represents Hashem's honor and glory in the world, bringing completeness and peace.