Two witnesses come and testify that they saw the new moon and therefore today is Rosh Chodesh (ex. Teves).
Two other witnesses come along and say that the first two witnesses couldn't possibly be telling the truth, since they (the 1st pair) were with them (the 2nd pair) and were not looking outside at the time they said they were.
The catch is, these latter two witnesses will be bar mitzva (I.e. eligible to testify) only on Rosh Chodesh Teves!
IF we believe the first pair of witnesses that today is Rosh Chodesh Teves, THEN the second pair of witnesses ARE bar mitzva and CAN testify against the first pair, in which case it is NOT Rosh Chodesh and if it is not Rosh Chodesh, they are NOT bar mitzva! If they are not bar mitzva, then we WILL believe the first pair of witnesses and THEN the second pair of witnesses ARE bar mitzva
Thank you
We do not receive such brilliant questions in the Kollel every day!
1) Your question is asked by the Minchas Chinuch in Mitzvah 4:15. He answers based on two rules: (a) there is a Torah Mitzvah to accept witnesses who saw the new moon, and (b) even if the witnesses were proved to be Zomemim, their testimony is acceptable b'Di'eved. On the basis of these two rules, the Minchas Chinuch argues that we cannot reject the testimony of the first pair -- who, we had assumed until now, were genuine witnesses -- because if we do not accept them, we will transgress the Torah Mitzvah to accept them.
2) I will explain more fully: The Minchas Chinuch aserts that since, when the first pair come at the beginning, we have no reason to suspect that there is any problem with them, it follows that we do not have to concern ourselves with possible future developments. This is because the Rambam (Hilchos Kidush ha'Chodesh 2:2) writes that the Din of the Torah is that we are not particular about the witnesses for the new moon. Even if Kidush ha'Chodesh was performed through witnesses who transpired to be Zomemim, the month is nevertheless sanctified. Therefore, even though the first pair automatically become invalid simultaneously with our accepting their testimony and thereby rendering the second pair as adults, this does not bother us at all. This is because there is nothing wrong with declaring Rosh Chodesh through invalid witnesses. The Minchas Chinuch writes that in this respect Kidush ha'Chodesh is different from other areas of Halachah.
3) The Minchas Chinuch writes (4:2 DH v'Lich'orah) that this is derived from, "This month is for you the first of months" (Shmos 12:2). Rashi writes, "ka'Zeh Re'eh v'Kadesh" -- you see the moon and proclaim the month as holy. We learn from this that there is a Torah Mitzvah upon Beis Din to accept witnesses and proclaim the new month. Since it is a Torah Mitzvah to accept the witnesses, it does not matter that later on they will be disqualified.
Kol Tuv,
Dovid Bloom