Gemara

(Rav Huna citing Rebbi): Saying 'Al Menas (on condition)' is like saying 'from now.'

(R. Zeira): In Bavel, we used to say that Chachamim argue with Rebbi about 'Al Menas'. In Eretz Yisrael, R. Yochanan said that all agree that saying 'Al Menas' is like saying 'from now.' They argue only about 'from today and after death.''

61a (Mishnah - R. Meir): A Tanai (stipulation) is invalid if it is not (Kaful, i.e. discusses both possibilities) like the Tanai made with the tribes of Gad and Reuven is invalid. "If they will cross the Yarden... if they will not cross to fight..."

R. Chanina ben Gamliel says, the verse does not teach this (it is not extra). There was a need to stipulate that if they do not cross, they will inherit in Eretz Yisrael.

Gitin 75a (Beraisa): If one said 'the Get is yours but the paper is mine', she is not divorced. If he said 'Al Menas that you return the paper to me', she is divorced.

Question: What is the difference between the two clauses?

Answer #1 (Abaye): The Beraisa is like R. Meir, who says that a Tanai is invalid unless it is Kaful. (In the Seifa, he did not double the Tanai, so it is Batel, and the Ma'aseh (the act about which he stipulates) takes effect.)

Answer #2 (Rava): In the Seifa, the Ma'aseh preceded the Tanai.

Answer #3 (Rav Ada bar Ahavah): In the Seifa, the Tanai and Ma'aseh involve the same matter.

Answer #4 (Rav Ashi): The Mishnah is like Rebbi, who says that saying 'Al Menas that' is like saying 'from now' (there is no contradiction between the Tanai and the Ma'aseh. She is divorced only if she fulfills the Tanai.)

Shmuel enacted that a Shechiv Mera (a sick person, who divorces due to concern lest he die) say 'if I do not die (from this illness), the Get is invalid. If I die, it is valid.'

Objection (Rava): Every Tanai must be like that of Benei Gad, in which the positive (if they will fight) preceded the negative (if they will not fight). Rather, he should say 'if I do not die, the Get is invalid. If I die, the Get is valid. If I do not die, it is invalid.' His initial words (if I do not die) are favorable. The positive stipulation (if I die) precedes the (repetition of) the negative.

Rishonim

Rif and Rosh (Gitin 37b and 6:10): A Tanai Kaful is needed only for a Tanai of 'if', like Benei Gad, and like Shmuel enacted for Get Shechiv Mera. It is not needed for a Tanai of 'Al Menas', for saying 'Al Menas' is like saying 'from now.'

Rosh: The primary opinion is like I wrote (see 6:9 below, that all Mishpetei Tanayim (conditions for a proper Tanai) are needed even for a Tanai 'Al Menas').

Rebuttal (of Rif - Hasagos ha'Ra'avad 1): When a Tanai Kaful is needed, a single Tanai is Batel and the Ma'aseh takes effect. When Kefel is not needed, the Tanai is valid and the Ma'aseh is Batel (whether or not it is like from now)! The Rif should have said that our only source to require Tanai Kaful is from Benei Gad, which was not 'Al Menas'.

Rambam (Hilchos Ishus 6:17,18): If one said 'from now' or 'Al Menas', he does not need a Tanai Kaful. When the Tanai is fulfilled, the Kinyan, Kidushin or divorce takes effect when the Ma'aseh was done, as if there was no Tanai at all.

Magid Mishneh: All the Ge'onim say so, for all the Mishnayos discuss Tanayim of 'Al Menas', and do not mention Kefel. Tosfos disagrees. The Ramban and Rashba were unsure. We follow the Kabalah of the Ge'onim.

Rambam (Hilchos Gerushin 9:1): If one divorced to take effect after a fixed time, she is divorced when the time comes, like a Tanai. However, here he did not divorce until the time, unlike divorce on Tanai, which takes effect immediately. Therefore, one who divorces on Tanai must make a Tanai Kaful, but one who divorces after a fixed time does not need Kefel or any Mishpetei Tanayim.

Rosh (Gitin 6:8): In other places, the Gemara did not explain that the Mishnah omitted the Mishpetei Tanayim. Everywhere else it relies on our Sugya (75a), which needed to discuss them to resolve the contradiction.

Rosh (9): Shmuel and Rava enacted a text with a Tanai Kaful. This shows that the Halachah follows R. Meir. It cannot be a mere stringency (i.e. the Halachah is unlike R. Meir), for this would lead to disaster! (If one omitted the Kefel, people will think that the Tanai is Batel and the Get is valid!) In our Sugya, elsewhere, and in a Tosefta, R. Meir requires a Tanai Kaful even with Al Menas. Some matters require only Giluy Da'as (revealing his intent), and some do not require even Giluy Da'as. The Rif says in a Teshuvah that 'Al Menas' is like 'from now' for Isur and monetary matters. Kefel is needed only when he says 'if'. Rav Hai Gaon and the Rambam agree. The Ra'avad explains that when the Tanai is from now, the Ma'aseh takes effect on this condition. A Tanai of 'if' comes to be Mevatel the Ma'aseh, so it must be Kaful. Rav Ashi teaches (unlike Abaye and Rava) that in the Seifa, the Tanai is valid, because he said 'Al Menas.' In this case, we do not follow R. Meir. The Ramban argues, for in a Tosefta R. Meir agrees that Al Menas is like 'from now,' yet he requires a Tanai Kaful. The Yerushalmi supports this. It says that a Tanai Kaful is not needed if he stipulated '(this will take effect) after...' or 'if...'

Ran (Gitin 37a Sof DH Tanu): Shmuel enacted Tanai Kaful for the Get of a Shechiv Mera, which is surely from now (a Get after death is invalid). The Rambam (Hilchos Gerushin 9:20) connotes that Shmuel enacted for a Get that is not from now; this is astounding. The Yerushalmi requires Kefel only for a Tanai 'Al Menas.' This is unlike the Bavli regarding a Tanai of 'if'. We should not say that it is totally contrary to the Bavli (i.e. that the Bavli requires Tanai Kaful for 'if', but not for Al Menas).

Rosh (ibid.): Also the Rambam does not require a Tanai Kaful if he stipulated 'after...' R. Chananel requires all Mishpetei Tanayim even when he said 'Al Menas.' Chachamim of our region agree, and this is the practice.'

Poskim

Shulchan Aruch (EH 38:3): If one said 'Al Menas' or 'from now' he does not need a Tanai Kaful.

Be'er Heitev (6,7): The Rashba (1125) says that if there is a Kinyan or a date, this is like a Tanai from now. Saying 'Al Tanai' is unlike saying 'Al Menas.'