Leviticus 11:27
And whatsoever goeth upon his paws, among all manner of beasts that go on all four, those are unclean unto you: whoso toucheth their carcase shall be unclean until the even. And whatsoever goeth upon his paws, among all manner of beasts that go on all four, those are unclean unto you: whoso toucheth their carcase shall be unclean until the even.
Among four-footed beasts, those that go upon paws are unclean, and whoever touches their carcass is unclean until evening. This rule belongs to the food and sanitary laws given through Moses that classified certain meats and animals as unclean and unfit for food. Those dietary limits, grouping unclean beasts with unclean birds and fish and with the prohibition on swine and other specified animals, were taught to Israel during the wilderness training before entering Canaan as part of God's care and training.
The prohibition was not arbitrary but was given to preserve life and health, because the forbidden meats were hurtful and not the best food. Eating them fills the blood with impurities and pollutes the blood and shortens life, while simple healthful food and pure blood preserve the faculties of mind and body and keep people clear and fit for God's service, so God specified the animals least objectionable for food. Swine's flesh in particular was forbidden because it fills the system with humors and in the warm climate of Israel often produced leprosy, acting with far more injurious influence there than in a colder climate, and swine were never designed to be eaten under any circumstances. Swine were instead useful as scavengers where herds ran free, devouring decaying substances on the ground that would poison the atmosphere and thus preserving health. If Israel had submitted to these wise prohibitions, God would have removed everything injurious to health and allowed no sickness among them, but departing from God's appointed diet brought loss, and when they would not submit He gave them what they lusted for.
Daniel and his companions refused the king's food rather than violate God's command against unclean meats, including swine's flesh, seeking to keep mind and body clear and fit for God's service. The regulations were later carried beyond their purpose when religious teachers added burdensome traditions and unwarranted extremes. They demanded that all water be strained lest the smallest unseen insect, classed with the unclean animals, be swallowed, which went beyond what God had required.