From Idolatry to Conversion: Judaism and Non-Jews: Idolatry and Non-Jewish Practices: Mixtures of Wool and Linen:
Why are mixtures of wool and linen forbidden?
The laws of Kilayim are "arbitrary" laws (chukim) that have no
"logical" rationales. In his Moreh Nevuchim, Maimonides suggests
that mixtures of wool and linen may have once been used in
garments worn by idol-worshipping priests, so that this specific
type of mixture was forbidden as an idolatrous practice. The
Torah forbids only mixing linen and sheep's wool; other mixtures
of fibers are permitted, even if one is an animal fiber and the
other is a plant fiber.
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