Bechoros Chapter 3, Mishnah 2. This Mishnah also deals with the practical aspects of purchasing animals when there is no certainty as to whether they have previously given birth or not. In the previous chapter, we saw cases where two animals were left with a non-Jew, and in the morning offspring were found there, and it is unknown which offspring belongs to which mother.
However, unusual situations can occur: an offspring nursing from an animal that is not its mother, an animal adopting an offspring that is not its own, or a cow, goat, and sheep beginning to nurse and produce milk even though they have never given birth. All of these are realistically possible, but they are not common at all.
The Rule of Rabban Shimon ben Gamliel:
From here emerges the simple point of the Mishnah, in the words of Rabban Shimon ben Gamliel: When a mother treats the offspring as her own, we can safely assume that it is indeed her offspring. We do not invent scenarios of doubt, but rather treat the situation as certain.
The Text of the Mishnah:
"Haloke'ach behemah meinikah min hanochri - eino choshesh shema benah shel acheres hi" - A Jew who buys an animal nursing an offspring from a non-Jew does not need to worry that she is not a mother at all, has never had offspring, and merely adopted this one. Even though it happens that sheep produce milk without having given birth, we do not follow this concern. We assume that this is her offspring and she is its mother, meaning she has already had offspring, and the next offspring will not be considered a bechor.
"Nichnas lesoch edro vera'ah es hamevachiros meinikos ve'es she'einan mevachiros meinikos - eino choshesh shema benah shel zo ba lo etzel zo, o shema benah shel zo ba lo etzel zo" - A person who enters his flock and sees mothers nursing their offspring, some that are giving birth for the first time and some that have given birth previously, and even though he did not actually see which offspring came from which animal - he does not need to worry about them getting mixed up.
One should not worry that the offspring nursing from the first-time mother is not her firstborn but another offspring she adopted, or that the offspring of the first-time mother went to nurse from the other one, resulting in a firstborn being designated by mistake. We follow what meets the eye: if an offspring nurses from a first-time mother, we assume that it is her firstborn.
In summary: In this Mishnah, Rabban Shimon ben Gamliel establishes that we do not worry about unusual scenarios of unnatural nursing or the adoption of offspring, neither when purchasing a nursing animal from a non-Jew nor when entering one's own flock. The rule is simple: we rely on what meets the eye.