I've always known that Jewish law forbids the wearing of garments made of cloth woven from two particular fibers, but I do not pay much attention to this prohibition. Like the preceding portion, Acharei Mot, Parashat Kedoshim continues to detail the set of laws that make up the Holiness Code. The wisdom of these laws seems obvious. But the parashah also contains other laws that we struggle with and even reject as Reform Jews, the most difficult of which is the prohibition against homosexuality. But even with all of this in mind, I have become fixated on verse 19, on laws concerning animals, planting, and clothes.
Jewish law prohibits the mixing of two kinds of anything.
An entire tractate of the Talmud is dedicated to the explanation of kilayim. The traditional reason for these restrictions posits that God is the Creator and human beings are not to engage in the creation of new species. They are 'closed' to one another by nature, and man has made them into kilayim, has brought them together in spite of their naturally not belonging together. The word sha-atnez is of unknown origin. The Mishnah explains that sha-atnez concerns the three processes that raw linen and wool have to undergo to become finished products.