GEMARA

(Mishnah - R. Meir): If he said ('if you will testify for me, Hash-m should not strike you', 'Hashem should bless you', or 'Hashem should do good to you (and the witnesses answered Amen, and did not testify)', they are liable;

Chachamim exempt.

36a - Question: R. Meir does not infer the positive from the negative (i.e. He should strike you if you do not testify), (or vice-versa)!

Answer #1: The opinions of R. Meir and Chachamim must be switched.

Answer #2: R. Meir does not infer the positive from the negative in monetary matters, but for Isurim, he does.

Question: Sotah is an Isur, and R. Tanchum said that R. Meir infers the negative from the positive, because it says "(if you are innocent) Hinaki (you will be safe)", and we can read this '(if you are guilty, Chanki (you will choke))''. Otherwise, R. Meir would not infer the negative from the positive!

Answer #1: Indeed, the opinions must be switched. R. Meir does not infer the negative from the positive even for Isurim.

Question (Ravina): If so, he should exempt one who serves in the Mikdash after drinking wine. A Mishnah teaches that he is liable (and no one disagrees!)

Answer #2: The opinions need (some texts - not) be switched. R. Meir does not infer the negative from the positive for anything involving money. He does infer for Isurim. Sotah is an Isur that involves money (the Kesuvah).

Gitin 75a: 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. 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.'

Bava Basra 137b (Rava): If one said 'take this Esrog Al Menas that you return it to me', and the recipient took it, he was Yotzei only if he returned it afterwards.

RISHONIM

Rambam (Hilchos Ishus 6:14): Some late Ge'onim require Kefel only for divorce and Kidushin, but not for monetary matters. One should not rely on this. Kefel and the other four Mishpetei Tanayim are learned from the Tanai with Benei Gad, and this did not involve divorce or Kidushin. The great early Ge'onim ruled like this (that also monetary matters require Mishpetei Tanayim). It is proper to do so.

Rambam (Hilchos Zechiyah 3:8): A Tanai 'Al Menas' is like 'from now.' It does not need Kefel. Some Ge'onim said so; I agree. My Rebbeyim required Kefel and putting the positive before the negative only for divorce and Kidushin. There is no proof for this.

Ra'avad: There is no proof, but there is a great reason. If someone gives land or sells 'only if you will do so and so', how can we make him give if his Tanai was not fulfilled?! However, for Gitin and Kidushin, we assess his intent. Since he decided to divorce or be Mekadesh, the Tanai is a mere taunt or jest. A Tanai does not help unless he strengthened it with Kefel and the other Mishpetei Tanayim, which are all to strengthen the Tanai. The inheritance of Benei Gad was not Muchzak to them like what a person inherited or bought.

Hasagos ha'Ra'avad (on Rif Gitin 37b): Some require Kefel only for divorce and Kidushin, but not for monetary matters. This is astounding! Tanai Benei Gad was monetary. If we learn from it, all the more so we need Mishpetei Tanayim for monetary matters!

Suggestion: Perhaps this is because d'Iy (what one will acquire, i.e. a Tanai for the future) does not acquire, and Tanayim of 'from now' do not require Kefel, for the Tanai of Benei Gad was not 'from now.'

Rejection (Ra'avad): The Tanai of Benei Gad was 'from now!' It says "Moshe gave to them!" One may not rely on this opinion.

Magid Mishneh (Hilchos Ishus 6:14) Teshuvas ha'Rif (31) says that surely these Ge'onim hold that R. Meir requires Kefel for monetary Tanayim, but the Halachah does not follow him. Letter of the law, we do not require Kefel for divorce, Kidushin or monetary matters. However, Shmuel enacted Kefel in the Get of a Shechiv Mera, lest a Beis Din hold like R. Meir and permit her even if the Tanai was not fulfilled. Therefore, if the Tanai was not Kaful and it was not fulfilled, we are stringent to consider her Safek divorced or Safek Mekudeshes. It seems that the Ramban and Rashba say so.

Ran (Kidushin 25b DH Rebbi): Surely, the Rif did not mean that we do not require any Mishpetei Tanayim. The Tanai must precede the Ma'aseh (like the Magid Mishneh proved), and a Shali'ach must be able to do the Ma'aseh.

Sefer ha'Zechus: We rely on the Ge'onim, who require Kefel only for divorce and Kidushin, but not for monetary Tanayim.

Ran (ibid.): The Torah taught about Kefel regarding monetary matters. Also, we find that R. Meir requires Kefel only for monetary matters, but regarding Isurim he infers the negative from the positive! Rather, surely it is a mere stringency to require Kefel for divorce and Kidushin.

Rosh (Bava Basra 8:48): Rava says that the Tanai about an Esrog is valid. Rava disagrees with Rav Ada, who says that the Tanai and Ma'aseh must involve different matters. The Rashbam explains that Kefel is needed only for divorce and Kidushin. Giluy Da'as (revealing his intent) suffices for monetary matters, in which we follow assessment of intent. R. Shimshon supports this from the Yerushalmi, which says that everywhere (else) R. Meir infers the negative from the positive, but here (divorce and Kidushin) he does not. He is stringent about Arayos. This is difficult, for R. Meir requires Tanai Kaful for other matters as well, in Bava Metzi'a (94a), Shevuos and Nedarim! R. Tam says that the Yerushalmi meant that he is more stringent about Arayos than other Isurim, such as Avodah while drunk. This is difficult, for it is a leniency to require a Tanai Kaful for divorce. Even if the Tanai was not fulfilled, she is divorced! Rather, the text of the Yerushalmi said 'everywhere (else) R. Meir does not infer the negative from the positive, and here he does not.' This is clearly a textual error. People 'fixed' this to say 'everywhere he infers, and here he does not.' Really, it should say 'everywhere he does not infer, and here he does.' It is a stringency of Arayos to be stringent about divorce and Kidushin when he did not double the Tanai (and it was not fulfilled). She does Chalitzah. Therefore, one must double every monetary Tanai. If he did not, the Tanai is Batel and the Ma'aseh takes effect. The Mishnayos that discuss Tanayim did not bother to write the Kefel, for obviously it is required. There are three levels in monetary matters. Sometimes, the intent is clear without even a Giluy Da'as, e.g. a document to divert property (a woman gives a gift to her son before marrying, so her husband will not get it) and Matanas Shechiv Mera (a sick person who fears lest he die. He intends to retract if he recovers.) Some things require Giluy Da'as, e.g. one who sells with intent to move to Eretz Yisrael. If there was no Giluy Da'as, we ignore his intent. He does not need a proper Tanai. Everything else needs a proper Tanai.

POSKIM

Shulchan Aruch (EH 38:4): Some say that we require the four Mishpetei Tanayim only when this is a stringency, but not if it is a leniency.

Beis Shmuel (6): This opinion holds that the Halachah follows Chachamim. We are stringent only about Gitin and Kidushin.