Mishnayos Bechoros Perek 5 Mishnah 4

היה בכור רודפו כו' הרי זה ישחט עליו – but the law only applies when he kicked it (i.e., the animal) at the time that of the chase [by the animal of the person], but not when it was not at the time of the chase.

הראויין לבוא בידי אדם – that one can say that a person make them (i.e., the blemishes) in it (i.e., the animal), such blinding its eye, cutting off his hand, or slitting its ear.

רועים ישראל נאמנים – a Jew who is shepherd of the cattle belonging to a Kohen is believed to state o the blemishes that befell the Kohen’s cattle that they happened on its own, and the Kohen, its owner can consume it with its blemish. But we do not suspect him that perhaps through a person this blemish came intentionally and that this shepherd is lying and he comes to permit it in order that his master the Kohen will slaughter it and feed him from it, for an Israelite/Jew is permitted to eat from the firstling with a blemish when the Kohen gives him [food] from it, for in this we do not suspect him, for a small amount of quaffing like this, we do not suspect that the shepherd would lie and transgress the sin in order for this small quaffing alone (see Talmud Bekhorot 35a – that they are so dependent upon their employers for their bread and butter).

רועים כהנים – shepherds who are Kohanim, are not believed when they shepherd the cattle of Israelites/Jews. For this Kohen is suspected that he himself placed a blemish in it, that this shepherd would say, My master did not leave it to me and I will give it to an other Kohen. And the same law applies that the Kohen shepherd is not believed to testify on a firstling of another Kohen, for we are suspicious of doing good to each other, for he held that I will testify for him now and he will do do me a favor when an Israelite gives me a pure firstling, and I will place a blemish in it and this Kohen will testify on my behalf that the blemish befell it on its own.

רבן גמליאל אומר נאמן הוא על של חבירו ואינו נאמן של עצמו – Rabban Shimon ben Gamaliel disagrees with the first Tanna/teacher, for he holds that the Kohanim were not suspected other than on the firstling itself for was already given to him by an Israelite, but on that of his fried, whether, his Master/teacher or another, they were not suspected concerning them.

רבי מאיר אומר כל החשוד בדבר לא דנו ולא מעידו – and there is a [difference] between the first Tanna/teacher and Rabbi Meir, for according to Rabbi Meir, a firstling belonging to a Kohen that blemish befell it, two from the marketplace have to testify upon it, but one witness, even one who is not a Kohen, does not help. But according to the first Tanna/teacher, especially a Kohen is not believed to testify on the firstling of a Kohen, for we are concerned for doing good to one another, but one who is not a Kohen, even one who testifies is believed to permit it. But Rabban Shimon ben Gamaliel states that even the children and the members of the household of a Kohen are believed, for specifically, only on what is his he is not believed. And his wife is also not believed because she is like is body. And the Halakha is according to Rabban Shimon ben Gamaliel.