Deuteronomy 14:11

Of all clean birds ye shall eat.

Of all clean birds may be eaten as part of directions that forbade unclean things and specified the animals least objectionable for food. The distinction between clean and unclean was not arbitrary authority but was given to preserve life and health. The flesh of forbidden creatures such as swine pollutes the blood and shortens life, while simple, healthful food keeps the blood pure and preserves the powers of mind and body. It gave no license to invent further rules, such as requiring water to be strained to avoid swallowing tiny insects. Later leaders carried the prohibition to unreasonable lengths, making eating and drinking a burden of ceremonies and restrictions that kept the mind under continual strain to discriminate between clean and unclean.