The prohibitions of kilayim and sha’atnez are divine statutes whose essential meaning lies beyond human understanding. Even so, the Sages throughout the generations have proposed various explanations for the mitzva. The central explanation is that wool and linen possess different qualities and each has its own purpose; God therefore commanded us to give each its proper place and not mix them (Ramban, Vayikra 19:19).
Wool and linen were the two principal materials used for clothing, yet they differ fundamentally and even stand in opposition. Wool comes from the animal world and alludes to the attribute of ḥesed, while linen comes from the plant world and alludes to the attribute of din; combining them is therefore complex and difficult. The Torah commanded that they be joined only in the context of a mitzva – in the priestly garments and in tzitzit – so that their union takes place in holiness and each power finds its proper expression, adding blessing and life. Moreover, since combining wool and linen is a mitzva in the priestly garments and in tzitzit, it is inappropriate to use this unique union for ordinary purposes, as it is reserved for holiness alone (Da’at Zekenim, Devarim 22:11; Ḥizkuni, Vayikra 19:19; Metzudat David of Radbaz §235; Likutei Halakhot, Kilei Begadim 1).
It has also been explained that these opposing forces can find their proper expression only in the context of a mitzva. When they are combined outside that context, their positive potential is constrained and prevented from emerging (Zohar III 86b). Within a mitzva, ḥesed and din are expressed properly; when mixed indiscriminately, however, the evil inclination turns them to harmful ends (Mikhtav Me-Eliyahu IV, p. 176). When a person ought to give tzedaka, for example, the evil inclination may cloak itself in din and argue that the poor person is undeserving. Conversely, when one must stand for truth and rescue someone being exploited by an oppressor, the evil inclination may cloak itself in ḥesed and find merit in the oppressor, thereby evading the obligation to help the victim.
It may be suggested that by separating wool and linen – the two natural fibers best suited for clothing – and permitting their union only in the context of a mitzva, the Torah taught humanity to develop additional fibers in which the human role in combining and shaping natural properties is more pronounced. Combining such fibers is therefore straightforward and carries neither a prohibition nor a mitzva.[12]
[12]. The Sages further taught that the root of the evil associated with combining wool and linen is alluded to in the offerings of Kayin and Hevel, which led to the first murder. The Torah recounts: “In the course of time, Kayin brought an offering to the Lord from the fruit of the ground. Hevel also brought from the firstborn of his flock and from their choicest parts. The Lord regarded Hevel and his offering, but Kayin and his offering He did not regard. Kayin became greatly distressed, and his face fell…. Kayin rose against his brother Hevel and killed him” (Bereishit 4:3–5, 8). The Sages identify Kayin’s offering from the ground with flax, and Hevel’s offering of a sheep, whose wool remained on it, with wool. We were therefore commanded not to combine the offering of the righteous with the offering of the wicked (Tanḥuma, Bereishit 9; Pirkei de-R. Eliezer 21).
It may be suggested further that had Kayin brought his flax offering on its own, it would have been accepted. That offering reflected his own spiritual level: he was a tiller of the soil and brought an offering from what he had grown. Once he saw Hevel bring a far superior offering – a sheep whose wool had not yet been shorn – he faced a difficult test. Would he humble himself and learn from his brother how to serve God with greater excellence, perhaps even asking Hevel to serve as his kohen? Or would he envy his brother for having found greater favor in God’s eyes and murder him? To protect us from such trials, the Torah commanded us not to combine these two materials except in the context of a mitzva (see Zohar III 87a).
