Chapter 26 — Part III: The Decalogue — The Third Commandment
Section 43. Why Animals Are Not To Be Employed On The Sabbath — To omit nothing that may interfere with the sanctification of the Sabbath, the Commandment mentions beasts of burden, because their use will prevent its due observance. If beasts be employed on the Sabbath, human labor also becomes necessary to direct them; for they do not labor alone, but assist the labours of man. Now it is not lawful for man to work on that day. Hence it is not lawful for the animals to work which man uses.