Kedoshim: Chapter 20
Mixed kinds and garments mixed of linen and wool (sha'atnez)

We learn that when God created the world he created a supernal force above every single thing, even every blade of grass. All the chieftains that have ever been designated as rulers have always behaved according to one supernal law, and the chieftains themselves are then called 'laws.' As each of them is appointed over a certain aspect of law, the mixture of species below uproots each force from its proper position and causes confusion above. When people behave as they should, they attract a Holy Supernal Spirit to them, but when they perform evil actions they draw a spirit from the side of evil. We are told why it is acceptable and proper to mix wool and linen in the Tzitzit but not elsewhere. Cain was a mixture from the Other Side, and therefore his sacrifice could not be mixed with that of Abel, who was of the same species as Adam and Eve, from the side of Holiness.