This:

The distinction of clean and unclean animals (see note on "Speakª° unto¹ the childrenª of Israel,ª saying,ª° Theseª [are] the beastsª which¹ ye shall eatª° among all¹¹ the beastsª that¹ [are] on¹ the earth.ª" {Lev 11:2}) existed even before the flood, though it probably then only related to sacrifices; but at this time we find there were very particular laws enacted respecting the diet of the people, and the ceremonial uncleanness contracted by touching the carcases of unclean animals. The reasons for the enactment of these laws seem to be:

1. As a test of obedience, and to teach the Israelites habits of self-denial, and the government of their appetites.
2. To keep them distinct from other nations, and consequently from their idolatrous usages, by throwing hindrances in the way of their social intercourse; for these distinctions were applicable both to persons and things. The Canaanites not only ate the animals prohibited by Moses, which we usually eat, but others also, among which dogs were one. Besides, many of those declared unclean were sacred among the heathen, and sacrificed to their gods.
3. Because those prohibited were innutritive and unwholesome; as the swine, the flesh of which being strong and difficult to digest, affords a very gross aliment, and produces, especially in hot climates, cutaneous, scrophulous, and scorbutic disorders, as the itch, leprosy, etc. "This¹ [is] the lawª of the burnt offering,ª of the meat offering,ª and of the sin offering,ª and of the trespass offering,ª and of the consecrations,ª and of the sacrificeª of the peace offerings;ª" {Lev 7:37} + "This¹ [is] the lawª for all manner¹ of plagueª of leprosy,ª and scall,ª" {Lev 14:54} + "This¹ [is] the lawª of him that hath an issue,ª° and [of him] whose¹ seedªª goethª° from¹ him, and is defiledª° therewith;" {Lev 15:32} + "This¹ [is] the lawª of the house;ª Upon¹ the topª of the mountainª the whole¹ limitª thereof round aboutª [shall be] most² holy.ª Behold,¹ this¹ [is] the lawª of the house.ª" {Ezk 43:12}