The Gemara (Bava Metzi’a 32b-33a) discusses the prohibition of tza’ar ba’alei chaim, causing pain to animals, and whether it is from the Torah (d’Oraisa) or from the Rabbis (d’Rabbanan), concluding that it is a dispute amongst the Tannaim - most notably, R’ Yosi HaGlili holds of the latter view. In that discussion, the foremost source for the view that it’s d’Oraisa is the obligation to unload one’s animal (Shemos 23:5), while R’ Yose HaGlili disagrees with that proof on grounds that we’re concerned with the owner’s loss, not the animal’s pain. The succeeding Gemara addresses various sources, but they all have to do with unloading animals.
- Bilam being rebuked for hitting his donkey (Bamidbar 22:32 - Moreh Nevuchim 3:17)
- Not muzzling an oz (Devarim 25:4 - Shitah Mekubetzes, Bava Metzi’a Ibid.)
- Just as HaShem is merciful to all creatures (Tehillim 145:9), so should we (follow in His ways, Devarim 28:9 - Chasam Sofer to Bava Metzi’a Ibid.)
We then have three different sources for the prohibition against tza’ar ba’alei chaim. How, then, can R’ Yosi HaGlili possibly hold that it’s not from the Torah? How does he respond to these sources?
It is noteworthy that the Sefer HaChinuch (550) uses Tza’ar Ba’alei Chaim as a reason behind the prohibition against plowing with two different animals. However, I did not include that in the question as he begins by quoting the Rambam’s reasoning, that one who plows with them together may come to crossbreed them, itself a prohibition. Presumably R’ Yosi HaGlili would explain similarly.