WHO WEARS TEFILIN ON THE RIGHT HAND? [Tefilin: Itar Yad]
(Mishnah): If one writes two letters he is liable, whether he used his right or left hand.
Question: Why is he liable for writing with the left hand? This is abnormal!
Answer #1 (R. Yirmiyah): The Mishnah discusses a left-handed person.
Objection: His left hand is like a regular person's Yemin (right or strong hand). He should be liable only for writing with his left hand!
Answer #2 (Abaye): The Mishnah discusses an ambidextrous person.
Answer #3 (Rav Yakov brah d'Vas Yakov): Our Mishnah is like R. Yosi, who is Mechayev for two letters due to marking.
Menachos 36b (Beraisa): (Tefilin are tied on) "Yadcha" - the left arm;
Yemin refers to the right - "Af Yadi Yasdah Eretz vi'Ymini Tipchah Shamayim", and Yad (Stam) refers to the left.
R. Yosi ha'Chorem says, sometimes 'Yad' refers to the right - "va'Yar Yosef Ki Yashis Aviv Yad Yemino"!
R. Noson says, we need not [learn from there]. "U'Kshartam...; U'Chsavtam" equates tying to writing (a Mezuzah). Just like we use the Yemin to write, we use it to tie. Therefore, Tefilin is tied on the left hand.
R. Yosi learns that Tefilin is tied on the left hand like R. Noson;
(Rav Ashi): Alternatively, R. Yosi learns from "Yadchah" written with a 'Hei', enabling us to read 'Yad Kehah' (the weak hand).
Tana'im argue about this;
(Beraisa #1): "Yadchah" teaches that the Shel Yad is tied on the left hand.
Others say, "Yadcha(h)" includes a Gidem (one missing his left hand).
(Beraisa #2): One missing his [left] arm is exempt from Tefilin [Shel Yad];
Others say, "Yadchah" includes a Gidem.
(Beraisa #3): An Itar (lefty) puts Tefilin on his right hand. This is his Smol (weak hand).
Contradiction (Beraisa #4): An Itar puts Tefilin on his left hand.
Resolution (Abaye): This discusses an ambidextrous person.
37b (R. Yitzchak): "V'Samtem... Al Levavchem... u'Kshartem" teaches that the hand Tefilin must be next to the heart.
R. Chiya brei d'Rav Avya would wear the Shel Yad even with his heart.
Rosh (Hilchos Tefilin 18): Sefer ha'Terumah says that if one writes with his left hand, but does all other tasks with his right hand, or vice-versa, some say that he puts Tefilin on his weaker hand, like Rav Ashi, who expounds "Yad Kehah." It does not depend on the hand he writes with.
Rashi (37a DH Mah): One writes a Mezuzah with his right hand, for most people do so. Therefore, also the right hand ties. Therefore, he must tie on the left hand.
Mordechai (Hilchos Tefilin 969): R. Elchanan asked, if one writes with his left hand, but does all other tasks with his right hand and foot, does it depend on writing, and he ties with the hand he writes with, or does it depend on strength? R. Peretz answered that he is like an ambidextrous person, and he puts on the left. However, for writing a Mezuzah, Rashi says that it depends on most people, and not on him. The Ri learns from Rashi that even a lefty puts on his left hand. The first Tana holds like this. Does the Halachah follow him? Perhaps Rav Ashi explains R. Yosi, but he does not hold like him. However, the Stam Beraisos hold like this. However, R. Yitzchak learns from "v'Samtem..." that Tefilin must be next to the heart. I.e. even a lefty must do so. We hold like him. However, one can reject this. Perhaps R. Yitzchak teaches that whichever arm one wears on, it must be even with the heart. A support is that the previous Tana'im and Rav Ashi did not learn the left hand from here. If not for Rashi, I would have said that R. Noson himself holds that a lefty puts on his right hand, since he writes with his left hand. We can say that they argue only about how to expound. In Shabbos, we answered that an ambidextrous person is liable with either hand. We did not say that the Mishnah is R. Noson or the first Tana! However, perhaps we preferred to establish the Mishnah like everyone. Alternatively, this is no Chidush according to them, for there is no source to exclude this.
Mordechai: However, training oneself to do things with his left hand does not matter. One is called a lefty only if he was born that way. We learn from Shabbos 103a. We did not establish the Mishnah do discuss one who trained himself to do things with his left hand. One can reject this. Perhaps this was the answer of an ambidextrous person! Ratz says that it depends on strength. If he trained himself even to write with his left hand, the Ri was unsure. One can explain R. Noson like I said. I say that he is like a lefty. The Ri does not consider him ambidextrous. I say that if they are not equally strong, he is a full lefty.
Shulchan Aruch (OC 27:6): If a lefty does all tasks with his left hand, he puts Tefilin on the right hand.
Mishnah Berurah (22): This is even if he trained himself to do so, and all the more so if it was due to sickness that weakened his hand. If he recovers and uses both hands equally, he is like most people.
Mishnah Berurah (23): The same applies if he does most tasks with his left hand.
Mishnah Berurah (24): If he put on his left hand, he was not Yotzei.
Shulchan Aruch (ibid.): If he is ambidextrous, he puts on his left hand.
Mishnah Berurah (25): If he can use either hand, but it is easier with his left, this is not called ambidextrous.
Shulchan Aruch (ibid.): If he writes with his left hand, but does all other tasks with his right, or vice-versa, some say that he puts Tefilin on his weaker hand, to fulfill "Yad Kehah."
Beis Yosef (DH v'Itar): Sefer ha'Terumah and the Mordechai learn from 'Yad Kehah' that it depends only on the weaker hand. Semak says that if a lefty can write also with his right hand, he is like an ambidextrous person. R. Yechiel agreed, for we learn from writing. Hagahos Maimoniyos (4:3) brought this. Even though it seems that the Rosh hold like Sefer ha'Terumah, the Tur favored R. Yechiel.
Mishnah Berurah (26): This opinion holds that there is nothing special about writing. It depends on the majority of tasks.
Shulchan Aruch (ibid.): Some say that the hand he writes with is called Yemin for this, and he puts Tefilin on the other hand.
Gra (DH v'Yesh): This is like R. Noson, and R. Yosi according to the first answer. Others hold unlike Rav Ashi. Even though the first Tana holds like Rav Ashi, in Beraisa #2 all hold unlike Rav Ashi. This is like Tosfos (37a DH Ein), who says that the Tana'im in Beraisa #2 do not argue. The first opinion [in the Shulchan Aruch] holds that they argue, and we hold like the first Tana [of Beraisa #2] and Rav Ashi. The Ri says that both [Tana'im in Beraisa #2] hold like Rav Ashi, just Others learn two laws. This is difficult. We said that Tana'im argue about [Rav Ashi's Drashah]! Also, this is only according to R. Yosi. The first Tana [of the first Beraisa brought] surely does not expound like R. Noson or "Yadchah." Surely, he holds that it depends on most tasks, and not just on writing. Also R. Noson and R. Yosi in the first answer need not say that it depends on how he writes. See Rashi. One could explain Rashi like the Mordechai did, but this is difficult. What is the source that R. Noson argues with the law of a lefty [and holds that all put on the left hand]? Rather, Rashi holds that it does not depend on writing alone [rather, on most tasks]. He was forced to say so, for R. Noson said "we need not [learn from there]." The way the Mordechai explains Rashi, R. Noson argues [about the Halachah] with the first Tana! Rashi is primary. Therefore, the first opinion [in the Shulchan Aruch] is primary.
Magen Avraham (10): The Bach says that if one does all tasks with his right hand, but writes with his left hand, all agree that he puts Tefilin on the left hand, for his right hand is primary. The argument is only about one who does all tasks with his left hand, and writes with the right. We rely on the Shulchan Aruch, unlike the Bach. However, if he was not born this way, rather, he trained himself to write with his left, but does all tasks with his right, we rely on the Bach, and he puts on his left hand.
Mishnah Berurah (27): If one cannot write, all agree that it depends on the hand he uses for other tasks.
Kaf ha'Chayim (31): If he uses one hand for writing, and the other hand for other tasks, each can be called his weak hand. Therefore, he does like most people, and puts on the left hand. However, the Rosh brings from Sefer ha'Terumah that it depends only on the weaker hand. Many bring from Mahari Tzemach that even a lefty must put on the left, lest he change the Supreme order. Also the Ari Zal says so. If one does all tasks with his left, he puts Tefilin on his left hand and blesses, for the Gemara says so. After Tefilah, he puts the Tefilin on his right hand, to fulfill Mahari Tzemach and the Ari Zal's words. He blesses without Hash-m's name, and says Keri'as Shema. If he writes or does other tasks with his left hand, or trained himself to become a lefty, Poskim argue about this, so the Kabalah decides the matter. He puts on his left hand and blesses. Also, since Poskim argue, he does like most people. Had the Beis Yosef seen the Ari Zal's words, he would have agreed. If he is stringent to put on his right hand after Tefilah, he will be blessed. Chesed l'Avraham says that he should wear Tefilin on both hands when saying Shema. This is wrong. Perhaps he transgresses Bal Tosif!
Ohr l'Tziyon (2:3:3): The Ari Zal said that a lefty wears Tefilin on his Smol, but this could mean his weak hand.
Note: Also Ben Ish Chai (Chayei Sarah 7) says that the Kabalah says that all wear Tefilin on the left side, but he did not bring this in the name of the Ari Zal.
Rema: This is the custom.