Thou shalt not plough with an ox and an ass - It is generally supposed that mixtures of different sorts in seed, breed, etc., were employed for superstitious purposes, and therefore prohibited in this law. It is more likely, however, that there was a physical reason for this; two beasts of a different species cannot associate comfortably together, and on this ground never pull pleasantly either in cart or plough; and every farmer knows that it is of considerable consequence to the comfort of the cattle to put those together that have an affection for each other. This may be very frequently remarked in certain cattle, which, on this account, are termed true yoke-fellows. After all, it is very probable that the general design was to prevent improper alliances in civil and religious life. And to this St. Paul seems evidently to refer, 2-Corinthians 6:14 : Be ye not unequally yoked with unbelievers; which is simply to be understood as prohibiting all intercourse between Christians and idolaters in social, matrimonial, and religious life. And to teach the Jews the propriety of this, a variety of precepts relative to improper and heterogeneous mixtures were interspersed through their law, so that in civil and domestic life they might have them ever before their eyes.

Thou shalt not plough with an ox and an ass together,.... They might be used separately, but not together; nor was it uncommon in some countries for asses to be employed in ploughing as well as oxen. Pliny (h) makes mention of some fruitful land in Africa, which when it was dry weather could not be ploughed by oxen, but after showers of rain might be ploughed by a mean little ass; so Leo Africanus (i) says, the Africans only use horses and asses in ploughing. The reason why they were not to be put together was either (as some think) lest the law should be broken which forbids the gendering of cattle with a divers kind, Leviticus 19:19 but Aben Ezra thinks the reason is, because the strength of an ass is not equal to the strength of an ox; and therefore he supposes this law is made from the mercy and commiseration of God extended to all his creatures; though perhaps the better reason is, because the one was a clean creature, and the other an unclean, and this instance is put for all others; and with which agree the Jewish canons, which run thus,"cattle with cattle, wild beasts with wild beasts, unclean with unclean, clean with clean (i.e. these may be put together); but unclean with clean, and clean with unclean, are forbidden to plough with, to draw with, or to be led together (k).''The mystery of this is, that godly and ungodly persons are not to be yoked together in religious fellowship: see 2-Corinthians 6:14.
(h) Nat. Hist. l. 17. c. 5. (i) Descriptio Africae, l. 2. p. 104. (k) Misn. Celaim, c. 8. sect. 2.

Thou shalt not plough with an ox and an ass together--Whether this association, like the mixture of seeds, had been dictated by superstitious motives and the prohibition was symbolical, designed to teach a moral lesson (2-Corinthians 6:14), may or may not have been the case. But the prohibition prevented a great inhumanity still occasionally practised by the poorer sort in Oriental countries. An ox and ass, being of different species and of very different characters, cannot associate comfortably, nor unite cheerfully in drawing a plough or a wagon. The ass being much smaller and his step shorter, there would be an unequal and irregular draft. Besides, the ass, from feeding on coarse and poisonous weeds, has a fetid breath, which its yoke fellow seeks to avoid, not only as poisonous and offensive, but producing leanness, or, if long continued, death; and hence, it has been observed always to hold away its head from the ass and to pull only with one shoulder.

An ox and an ass - Because the one was a clean beast, the other unclean whereby God would teach men to avoid polluting themselves by the touch of unclean persons or things.