God created distinct species in the world, and in several contexts commanded us not to mix them with one another. The Torah states: “You shall keep My statutes: You shall not breed your animal with a different species; you shall not sow your field with kilayim; and a garment of kilayim, sha’atnez, shall not come upon you” (Vayikra 19:19). It also says: “You shall not sow your vineyard with kilayim, lest the entire yield become forbidden – the seed you sow and the produce of the vineyard. You shall not plow with an ox and a donkey together. You shall not wear sha’atnez, wool and linen together” (Devarim 22:9–11). This category of prohibitions is generally called kilayim, a term meaning “mixture.” In other words, even when each species is permitted on its own, their combination is forbidden. Therefore, the prohibition against cooking meat with milk is also included among the prohibitions of kilayim.
There are seven prohibitions of kilayim: (a) animal kilayim – breeding two different animal species; (b) working two different animal species together; (c) tree kilayim – grafting two different plant species; (d) seed kilayim – sowing two types of seeds together; (e) vineyard kilayim – sowing grain or vegetables near grapevines; (f) garment kilayim – wearing clothing that contains wool and linen together; and (g) meat and milk.
The prohibition of kilayim applies to both plants and animals. In some forms of kilayim, only the act of mixing is prohibited. After the fact, however, one may eat or derive benefit from the result of the mixture. This is true of seed kilayim, tree kilayim, and animal kilayim, whether through breeding or labor. By contrast, in other forms of kilayim, the act of mixing is prohibited, and the resulting product is forbidden for consumption or benefit. This is true of vineyard kilayim and cooking meat with milk. In the case of garment kilayim (sha’atnez), the prohibition is only to wear such a mixture; there is no prohibition against making it for a gentile or for a purpose other than wearing.
Mitzvot dependent on the Land, such as seed kilayim and vineyard kilayim, apply by Torah law only in Eretz Yisrael, within the territory settled by those who came up from Egypt (below, chapter 12, note 2). By contrast, mitzvot that are not dependent on the Land, such as garment kilayim, meat and milk, and animal kilayim, apply in the Diaspora as well. The prohibition of tree kilayim – grafting one species onto another – also applies in the Diaspora. Although this prohibition appears to be dependent on the Land, the prohibited act takes place above ground, not in the ground itself (below, section 5). Since the law of vineyard kilayim is especially stringent, as the mixture is forbidden both for consumption and benefit, the Sages decreed that it is prohibited in the Diaspora as well (below, chapter 4, section 4).
Although we cannot grasp the full depth of the mitzvot, which are divine in origin and rooted far beyond human understanding, some mitzvot have reasons that are more readily understood. These are called mishpatim, or ordinances. Others have reasons that are less understandable to us; these are called ḥukim, or statutes. The mitzva of kilayim belongs to the category of ḥukim, as the verse says: “You shall keep My statutes” (Vayikra 19:19). Rashi likewise explains that these are decrees of the King whose reasons are not revealed. We may suggest that these ḥukim reflect the unique qualities that God implanted within all created beings, qualities that point to the distinct identity and purpose of each creation. The divine statute, whose secret we cannot fully fathom, concerns the meaning and unique purpose of every species God created. Through the prohibition of kilayim, we are commanded to preserve the special and dignified place of every species, not to blur its uniqueness by mixing it with another species (Ramban and Maharal to Vayikra 19:19; Sefer ha-Ḥinukh 244). Similarly, the Sages said: “‘You shall keep My statutes’ – the statutes that I already engraved for you in the nature of creation, you must now observe” (Kiddushin 39a).
The word kilayim is related to kele, which means prison. The mixture imprisons the distinctive qualities of the species, preventing them from being expressed properly once they have been mixed with another species (Zohar III 86a; Rabbeinu Baḥya). With seed kilayim and vineyard kilayim, the prohibition applies even when the species merely grow near one another, even though the species themselves do not actually mix or merge. These prohibitions apply specifically in Eretz Yisrael because in the Land, the sanctity of creation is more fully revealed. As a result, the uniqueness of each species is more apparent there, and its special and dignified place must be preserved.[1]
[1]. Rashi to Vayikra 19:19 emphasizes that the mitzva of kilayim is a ḥok, a statute whose reason is not revealed. By contrast, Ramban writes there that kilayim has an important rationale: God commanded us to preserve each species in the form in which He created it, “so that they never change.” One who grafts species together undermines and damages the quality of the species and diminishes the honor of the Creator, as though implying that the Creator did not complete His world properly. The Sages similarly said: “There is no blade of grass that does not have a mazal, a heavenly force, striking it and saying to it, ‘Grow’” (Bereishit Rabba 10:6). One who mixes species damages the root of their mazal. Additionally, such hybrids do not reproduce.
Maharal challenges this explanation (Gur Aryeh to Vayikra 19:19). After all, the Sages said that everything God created during the six days of creation requires improvement, such as wheat, which must be ground and baked; yet we do not say in such cases that God left His world incomplete (Bereishit Rabba 11:6). Similarly, the Sages said regarding brit mila that a person is born uncircumcised, and his completion comes through the one who fulfills the covenant (Tanḥuma, Tazri’a 5). Nevertheless, regarding crossbreeding animals and grafting trees, Maharal agrees with Ramban. In these cases, one changes the statutes of the world by joining separate species together. Ramban may also agree that human beings were created to develop and perfect creation. It may even be permitted to create different species through hybridization, as explained below, section 5. However, one who cultivates different species must give each species its proper place and not damage it by mixing it with another species. This may also be the intention of the Sages and Rashi, as explained above.
