The Mishnah discusses the laws of substitution, known as temurah, specifically regarding consecrated animals. It states that one cannot substitute a consecrated animal for a non-consecrated one, nor a non-consecrated animal for a consecrated one. This prohibition applies whether one attempts to substitute a good animal for a bad one, or a bad animal for a good one. The Gemara then delves into the precise meaning of "good for bad" and "bad for good."
The case of "good for bad" refers to an animal that was initially consecrated while it was unblemished (tamim), but subsequently developed a blemish (ba'al mum). In such a scenario, the animal retains its consecrated status despite the blemish. The Mishnah forbids attempting to substitute this blemished consecrated animal for another, unblemished animal. The Gemara asks for the source of this understanding, specifically how we know that "bad" refers to an animal that became blemished after its consecration, rather than one that was inherently bad from the outset (ra mi'ikaro) or blemished before consecration (kadam mum le'hekdesh).
The Gemara explains that the Torah's phrasing, "He shall not exchange it, nor substitute it, a good for a bad, or a bad for a good," contains a crucial redundancy. If the Torah merely intended to forbid substituting any consecrated animal, regardless of its initial state, it could have simply stated, "He shall not exchange it, nor substitute it, for a bad animal." The inclusion of both "good for bad" and "bad for good" indicates a more nuanced meaning. The phrase "good for bad" teaches us about an animal that was initially good (unblemished) when consecrated, but later became blemished. Despite its current blemished state, it is still considered "good" in the context of its original consecration. The prohibition of temurah applies to such an animal, and if one attempts to substitute it, the substitute animal also becomes consecrated, and the person incurs the punishment of lashes (malkut).
Rava offers an alternative interpretation of the same verse. He agrees with the halachic outcome but derives it differently. According to Rava, the words "good" in the phrases "good for bad" and "bad for good" are superfluous if the Torah's only intent was to forbid substituting any consecrated animal. He argues that the word "bad" alone would suffice to teach that it is forbidden to substitute a non-consecrated animal for a blemished consecrated animal. The extra word "good" comes to teach two distinct laws. Firstly, it teaches that even substituting an unblemished animal for another unblemished consecrated animal (tamim le'tamim) is forbidden and incurs malkut. Secondly, it teaches that only an animal that was unblemished at the time of its consecration (tov mi'ikaro) and subsequently became blemished can be subject to temurah. An animal that was blemished from the outset (ba'al mum mi'ikaro) cannot be subject to temurah.
The Gemara then challenges Rava's first derivation, arguing that the prohibition of "unblemished for unblemished" could be learned through a kal v'chomer (an a fortiori argument). If substituting a blemished animal for an unblemished one, which is an "upgrade," incurs malkut, then certainly substituting an unblemished animal for another unblemished one, which is an "equal exchange," should also incur malkut. The Gemara responds that while a kal v'chomer can establish a prohibition, it cannot establish the punishment of malkut. Therefore, a specific verse is needed to teach that "unblemished for unblemished" incurs malkut.
The Gemara then examines the double language in the verse: "He shall not exchange it (lo yachalifenu) nor substitute it (v'lo yamir oto)." The Sages (Rabbanan) interpret this double language as referring to two different scenarios. "He shall not exchange it" (lo yachalifenu) refers to substituting one's own consecrated animal for an animal belonging to someone else. "Nor substitute it" (v'lo yamir oto) refers to substituting one's own consecrated animal for another animal that one owns. The Gemara clarifies that even if one attempts to substitute one's consecrated animal for another person's non-consecrated animal, the substitute animal becomes consecrated, and the person incurs malkut. This is only possible if the owner of the non-consecrated animal gives permission for the substitution, as one cannot consecrate something that does not belong to them.
The Mishnah continues by discussing the quantity of animals involved in temurah. It states that one can substitute one non-consecrated animal for two consecrated animals, or two non-consecrated animals for one consecrated animal. Similarly, one can substitute one non-consecrated animal for a hundred consecrated animals, or a hundred non-consecrated animals for one consecrated animal. In all these cases, the substitute animal(s) become consecrated. This is the view of the Tanna Kamma (the first Tanna). Rabbi Shimon disagrees, stating that substitution can only occur "one for one," meaning one non-consecrated animal for one consecrated animal. He derives this from the verse that states, "an animal for an animal" (behemah bi'vehemah), interpreting "animal" in the singular to mean a single animal.
The Sages (Rabbanan) counter Rabbi Shimon's argument by pointing out that the word "animal" can sometimes refer to multiple animals, as seen in the Book of Jonah, where "many animals" (behemah rabbah) is used. Therefore, the singular form "animal" does not necessarily imply a one-to-one exchange. Rabbi Shimon then clarifies his reasoning, stating that his primary source for the one-to-one rule is a different verse, but he could also interpret "animal for an animal" in the singular if he chose to. The Gemara then asks if, according to Rabbi Shimon, one can make multiple "one-to-one" substitutions consecutively from the same consecrated animal. For example, if one has a consecrated animal and substitutes it for a non-consecrated animal, and then takes the original consecrated animal and substitutes it for another non-consecrated animal, and so on. Rabbi Yochanan states that this is indeed permissible, as each act of substitution is a distinct one-to-one exchange.
The Gemara then presents a complex case to clarify the laws of temurah further. Imagine an animal designated as a sin-offering (korban asham). This animal becomes blemished (ba'al mum). According to halacha, a blemished sin-offering cannot be sacrificed, so it is redeemed, and a new unblemished animal is brought as a substitute sin-offering. This second animal is then lost. A third animal is brought as a sin-offering in its place and sacrificed. Subsequently, the second animal, which was lost, is found. This found animal is no longer considered a sin-offering but becomes a burnt-offering (olah). The question arises: Can one now perform temurah on this found animal, which is now a burnt-offering, even though it originally received its consecrated status as a substitute for the first sin-offering, on which temurah had already been performed?
Abaye analyzes this complex question, suggesting that the intricate scenario presented is unnecessary to arrive at the core halachic doubt. He breaks down the question into two simpler possibilities. If the question is about two distinct animals (shnei gufim) but a single consecrated status (kedushah achat), then a simpler case would suffice: a sin-offering is designated, temurah is performed on it, and the original sin-offering becomes blemished. Can temurah be performed again on the substitute animal? If the question is about a single animal (guf echad) that has acquired two different consecrated statuses (shte kedushot), then a different simple case would suffice: a sin-offering is designated, it gets lost, another sin-offering is brought in its place and sacrificed, and then the original lost sin-offering is found. It is now a burnt-offering. Can temurah be performed on this original animal, which now has a different consecrated status?
The Gemara concludes that the original complex question was indeed necessary because it combines both elements: two different animals (the second and third animals) and two different consecrated statuses (the second animal, initially a sin-offering, becomes a burnt-offering). The question is whether the act of temurah performed on the first sin-offering prevents further temurah on any animal that derives its consecrated status from it, even if it's a different animal or has a different consecrated status. The Gemara leaves this question unresolved (Tiku).