Siman 318

A ewe that has not previously given birth and bore two males, even if both their heads emerged as one, nevertheless both are not sanctified as a firstborn. The reason is that we hold that it is impossible to be exact in matters of Heaven; meaning, even if we say that when a person intends to make two equal parts it is possible to be exact such that it will be so, nevertheless in matters of Heaven—meaning that it should occur so on its own—it is impossible. Therefore, both did not emerge equally and one preceded the other, but our eyes are too limited to perceive this. Therefore, only one of them is a firstborn. Consequently, since the kohen is the one seeking to extract and the owner is the one in possession, the kohen takes the lean one. Even though there is a reasoning to say that the healthy one emerges first because with its strength it pushes the other aside, and there is one who holds so in the Gemara (18a), nevertheless the halacha is not so, because this is not a clear matter. The second one, which is in the hand of the owner, is a doubtful firstborn and is eaten by the owners when it has a mum. If one of them died, the kohen has nothing here, for the burden of proof is upon the claimant. So too, if it bore a male and a female, the kohen has nothing here, for perhaps the female emerged first, and the male is eaten by the owners when it has a mum according to the law of a doubtful firstborn (regarding this matter that it is impossible to be exact, I have explained according to the words of the Tosafot in Bekhorot 17a, see there).

It is simple that any case of a doubtful bekhor that remains in the possession of the owner is forbidden for shearing and labor, and he is also obligated to give the priestly gifts from it to a kohen. For from whatever side you look at it: if it is a bekhor and exempt from gifts, then the whole of it belongs to the kohen; and if it is not a bekhor, it is liable for gifts, as was written in siman 61. And one should not say that if so, according to this, the kohen could say to him, "I certainly have a connection to this offspring, either the whole of it or part of it, and you perhaps have no connection at all—such as if it is a bekhor—and therefore a doubt does not remove a certainty, and I shall take the whole of it," like that case of a doubt and a levir regarding the grandfather's property in Yevamot (38a), where the levir takes all the property because part of it is certainly his, see there. Here too it is the same, for it is not comparable; for there, by virtue of the fact that he takes the part, he takes the whole. But the kohen, regarding the part that he takes, does not know himself for what reason he takes it—whether because of gifts or because of primogeniture—and therefore he too is not considered a certainty. It is like what was previously mentioned there (37b) regarding a doubt and the sons of the levir who came to divide the property of the deceased, where for this reason we do not consider the doubt as a certainty, see there.

Two ewes that had not previously given birth and gave birth to two males, both belong to the kohen, and there is no novelty in this (see Tosafot Yom Tov, chapter 1, mishnah 3). A male and a female, the male belongs to the kohen. Two males and a female, the kohen takes the lean one and the second is a doubt; and if one of them died, the kohen has nothing, for this living one is a doubtful bechor, and the burden of proof is upon the claimant. If they gave birth to two females and a male, or two males and two females, the males are a doubt, for I say each one gave birth to a female and a male and the female emerged first; therefore, there is nothing here for the kohen, for the burden of proof is upon the claimant. If one had previously given birth and one had not previously given birth, and they gave birth to two males, one is for him and one is for the kohen, and each of them is a doubtful bechor, and the kohen takes the lean one; if one of them died, there is nothing here for the kohen, for this living one is a doubt. And likewise, if they gave birth to a male and a female, there is nothing here for the kohen, for this male is a doubtful bechor. And likewise, a convert who converted (in ancient days), and it is a doubt whether his animal gave birth before he converted or a doubt whether after, it is a doubt and shall be eaten by the owners upon its blemish. And the general rule is: any that is a doubtful bechor shall graze until it sustains a blemish and shall be eaten by its owners. And any doubtful bechor that the kohen seized, according to the opinion of the Rambam, we do not remove it from his hand, and according to the opinion of the Rosh, we do remove it. And every doubtful bechor is liable for matanot, and this is according to the opinion of the Tur; but according to the Rambam, this is only when the kohen took nothing, but when there were two and the kohen took one, the second is exempt from matanot on the grounds that it is as if the kohen granted him the second one, and it is a dispute in the Gemara (18b). And consequently, even seizure is of no avail according to the Rambam when the kohen took one (and with this which we have explained, you will understand the words of the Shach and the Taz in subsection 1 which contradict one another, and there is no contradiction here, for the Taz wrote according to the Tur and elaborated on this in section 1, and the Shach wrote according to the Rambam, and delve into this).