Aurel, thank you very much for drawing my attention to this very interesting Ritva.

1. The Ritva writes that his source that it must be Shabbos- related is logic -- it is "Mistabra." The Ritva explains that his logic is that if it is possible to do the measuring after Shabbos, then it should not be permitted on Shabbos itself. In other words, measuring should not be permitted on Shabbos if no Mitzvah is involved. Therefore, since a Heter is needed for measuring, why should one rely on a Heter if he can do the measuring just the same after Shabbos?

2. The Ritva writes that another reason for why measuring is permitted only if it is needed for Shabbos itself is that it involves an action. Measuring or weighing is a more significant activity than merely thinking or walking. This is the way in which the Ritva distinguishes between weighing and measuring, which may be done on Shabbos only for the purpose of a Mitzvah, and the law of the Gemara earlier (150a) which states that one may make calculations on Shabbos for the purpose of a Mitzvah even when the benefit of those calculations will be derived only after Shabbos. (The Mishneh Berurah 206:26 writes that one may make a mental reckoning of how much he requires for a Se'udas Mitzvah. According to the Ritva, this is also permitted for a meal being held after Shabbos.)

3. I found support for the Ritva's opinion in Teshuvos Mahari Bruna #46 (one of the early Acharonim, as cited by the Tzitz Eliezer 9:18, who describes him as "one of the great Poskim"). The Mahari Bruna writes that when the Mishnah (157a) states that one may nullify on Shabbos only vows which are needed for Shabbos, it implies that the law mentioned next to this one in the Mishnah -- that one may measure a Mikvah to ensure that it contains 40 Se'ah, applies only when this is necessary for eating on Shabbos, which is a Mitzvah.

4. In fact, the Tiferes Yisrael on the above Mishnah (#32) also appears to agree with the Ritva and the Mahari Bruna. He writes that one may measure the Mikvah to ensure that it contains 40 Se'ah since it is then fit for Tevilah on Shabbos. This implies that measuring for a Mitzvah which is not needed on Shabbos is not permitted.

5. In summary, possible reasons for why the Ritva requires a "Shabbos purpose" include: (a) if it is possible to measure after Shabbos, there is no need to rely on a Heter to do this on Shabbos itself; (b) measuring is a positive activity and therefore its laws are more stringent than merely thinking (or walking); (c) since measuring the Mikvah is mentioned in the Mishnah together with "Hafaras Nedarim," once the Gemara (157a) states that Hafaras Nedarim is permitted only for needs on Shabbos, it follows that measuring is also permitted only if it is for the needs of Shabbos.

6. You asked what Shitah disagrees with the Ritva and maintains that one may measure for a Mitzvah even though the Mitzvah is after Shabbos. I hope to address this question in a later reply.

Kol Tuv,

Dovid Bloom

I am now going to attempt to find the view that disagrees with the Ritva.

1. I must admit that when I first saw the Ritva, I thought it was going to be very easy to find a Shitah that disagrees with him, but as I went further into the Sugya I realized that it is not so simple, as I will explain, bs'd.

2. The Mishnah in Shabbos (157a) states that one may measure a Mikvah on Shabbos. The Mishnah then cites an incident in the days of Rebbi Tzadok's father and concludes that one of the things we learn from their words is that one may measure a Mikvah on Shabbos. Rashi (DH u'Modedin) writes that the measuring is performed in order to learn a matter of "Hora'ah" -- how to issue a Halachic ruling.

3. The simple reading of Rashi appears to be that the measurement does not necessarily have to be done for use on Shabbos itself. The emphasis in Rashi seems to be that one is measuring in order to learn Halachah, and since this is clearly a Mitzvah, there is no condition required that one use the information gained on Shabbos itself.

4. In addition, upon looking in the Shulchan Aruch (OC 306:7) we do not seem to find anyone who says that the measuring must be done for use on Shabbos. The Shulchan Aruch simply states that one may measure on Shabbos if this is done for the sake of a Mitzvah. The Mishnah Berurah (306:34) points out that this Halachah seems to contradict the general rule in Hilchos Shabbos that rabbinical prohibitions are not relaxed for the sake of performing a Mitzvah. The Mishnah Berurah answers that the Halachah for measuring is different because the "Isur" is not so strong, as it is merely "Uvda d'Chol" -- it is a weekday activity, and therefore it is permitted in order to do a Mitzvah. Again, the Mishnah Berurah (306:35) stresses that if the measurement is done for the sake of a Mitzvah, it does not appear to be a weekday activity and therefore is permitted.

5. From the above sources it appears that the measurement need not be done for a Shabbos purpose, because as long as it is a Mitzvah it does not appear to be a weekday activity.

6. However, I found (with the help of a search of the Otzar ha'Chochmah database) that the Ritva elsewhere in Shas also writes that the measuring is permitted only when done for Shabbos purposes. In Eruvin (42a), where the Gemara states that if one is walking and does not know where his Shabbos boundary ("Techum") ends, one may take 2000 average paces and this is one's "Techum Shabbos." The Ritva there writes that one may perform such a counting even on Shabbos if one needs to go somewhere "l'Shum Tzorech Shabbos" -- "for any Shabbos need." He adds that one counts the paces to oneself, and this is considered a measurement of a Mitzvah which is permitted, as stated at the end of Maseches Shabbos.

7. The Ritva appears to be consistent with his opinion in Shabbos 157b. (Although in Shabbos 157b he writes merely that it is likely that one requires a Shabbos purpose, in Eruvin this seems clear to him without any doubt.) The Ritva is cited by the Mishnah Berurah OC 397:5 and Sha'ar ha'Tziyun #3.

8. I spoke about this matter this week with my Rebbi, Rav Yehoshua Neuwirth, shlit'a (author of Shemiras Shabbos k'Hilchasah). He said that since the Mishnah Berurah cites the Ritva, this means that the Halachah is that measuring for a Mitzvah is permitted only if it has a Shabbos purpose.

Kol Tuv,

Dovid Bloom