R. Joshua of Sikhnin said in the name of R. Levi, “There are four things that the evil drive would refute [as irrational], and for each of them is written [the word,] huqqah (i.e., an unquestioned statute). Now these concern the following: (1) the nakedness of a brother's wife, (2) diverse kinds, (3) the scapegoat, and (4) the red heifer.” In regard to the nakedness of a brother's wife, it is written (in Lev. 18:16), “You shall not uncover the nakedness of your brother's wife”; [yet if the brother] dies without children [it is written] (in Deut. 25:5), “her brother-in-law shall have sexual intercourse with her [and take her for a wife].” And it is written about the sexual prohibitions (in Lev. 18:5), “And you shall keep [all] My unquestioned statutes [...].”
In regard to diverse kinds, it is written (in Deut. 22:11), “You shall not wear interwoven stuff, [wool and flax together]”; yet a linen cloak with [wool] tassels is permitted. And for [this commandment also] it is written, [that it is] an unquestioned statute. [Thus it is written (in Lev. 19:19),] “You shall keep My unquestioned statute.
You shall not mate your cattle with a different kind…, [nor shall you wear a garment with diverse kinds of interwoven stuff].” In regard to the scapegoat, it is written (in Lev. 16:26), “And the one who sets the azazel-goat free shall wash his clothes”; yet it is [the goat] itself that atones for others. And for [this commandment also] it is written (in Lev. 16:34), “And this shall be to you an unquestioned statute forever.”
In regard to the red heifer, where is it shown? Since we are taught (in Parah 4:4), “All engaged with the [rite of the red] heifer from beginning to end render [their] garments unclean”; yet it is [the heifer] itself that purifies [what is] unclean. And for [this commandment also] it is written, [that it is] an unquestioned statute. Thus it is written (in Numb. 19:2), “This is an unquestioned statute of the Torah.”