You shall not eat of anything that dies of itself: you shall give it unto the stranger that is in your gates, that he may eat it; or you may sell it unto a foreigner: for you are a holy people unto the LORD your God. you shall not boil a kid in its mother's milk.
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George Leo Haydock
Of itself, or by suffocation.
Stranger, who has not embraced your religion. (Menochius)
Hence it is inferred, that the Jews might keep unclean animals, and sell them; as they did not defile till they were dead. (Jansenius)
If they had been unclean by nature, they could not have been sold, which shows that this ceremonial law regarded only the Jewish religion.
Dam. All appearance of cruelty must be avoided. Christ, who is signified by the kid, on account of his assuming our sinful nature, shall not be slain in his infancy. (St. Thomas Aquinas, i. 2. q. 102. a. 6.) (Worthington).
Some take this prohibition literally, and extend it to calves and lambs. The Arabs use milk in almost all their ragouts. (Roger. ii. 2.)
Others think that kids must not be eaten, while they are as yet too tender, Qui plus lactis habet quam sanguinis. (Juvenal, Sat. xi.)
But we believe that God forbids the paschal lamb or kid to be offered while it sucks. It must be of a competent age, of one year, Exodus xii. 5., and xxiii. 19. Other victims would do if they were only eight days old, Leviticus xxii. 27. (Calmet)