Mixed kinds

Bacher and Eisenstein define sha'aṭnez as fabric consisting of a mixture of wool and linen. They state that wearing it is forbidden by the Mosaic law, citing Leviticus 19:19 and Deuteronomy 22:11.

Wilhelm Bacher and Judah David Eisenstein, The Jewish Encyclopedia, s.v. 'Sha'aṭnez' (New York: Funk & Wagnalls, 1901-1906) — Opening definition of sha'aṭnez

Augustine replies that Faustus shows ignorance of the difference between moral and symbolical precepts. He gives “Thou shalt not covet” as a moral precept and “Thou shalt circumcise every male on the eighth day” as a symbolical precept.

Augustine of Hippo, Reply to Faustus the Manichaean (Contra Faustum), Book VI (trans. Richard Stothert), in Nicene and Post-Nicene Fathers, ser. 1, vol. 4 — Book VI (reply to Faustus' attack on the precepts of the Old Testament)

Haydock holds that these precepts were to be literally observed. Yet they concealed a moral instruction of the greatest consequence: all unnatural intercourse was to be avoided.

George Leo Haydock, George Haydock's Catholic Bible Commentary (1859), on Leviticus 19:19 (bibliaplus.org edition) — Comment on Leviticus 19:19 (with Calmet and Tirinus)

The source urges readers to distinguish rules belonging to the law of nature from rules added later. It says the law includes the ten commandments spoken by God before Israel’s idolatry, while the further bonds laid on Israel after sin should not be drawn upon oneself.

Anonymous (Constitutions of the Holy Apostles), Constitutions of the Holy Apostles (Didascalia Apostolorum), in Ante-Nicene Fathers, vol. 7 (eds. A. Roberts and J. Donaldson) — Book I ('Concerning the Laity'), Sec. II ('Commandments to Men'), ch. VI ('Abstain from all the heathen books')

Calvin takes these four precepts, which all condemn strange medleys, to be supplements of the First Commandment. The reason added in Deuteronomy directs us to this, he says, where God declares that the produce of the seed and vineyard is polluted if there are diverse mixtures.

John Calvin, Harmony of the Law - Volume 2 (Commentaries on the Four Last Books of Moses, trans. Charles William Bingham; CCEL) — Comment on Deuteronomy 22:9-11 (the precepts against strange medleys; cf. Leviticus 19:19)

Wesley says the prohibition had several purposes. It was partly to restrain human curiosity and boldness in trying to amend or change God's works, partly to teach men by restraint even on brute creatures to abhor all unnatural lusts, and partly to teach Israelites to avoid mixtures with other nations in marriage or religion.

John Wesley, Explanatory Notes upon the Old Testament - Notes on the Third Book of Moses called Leviticus (Wesley Center Online text) — Note on Leviticus 19:19