Gemara

28a: If one was forced (by Nochrim) to eat Matzah, he was Yotzei.

Rava: (He was Yotzei even though he did not intend.) This teaches that one who blows a Shofar for a song is Yotzei (if he heard the right notes! Some texts attribute this to Rabah.)

Question: Obviously these are the same! What is Rava's Chidush?

Answer: One might have thought that there he is Yotzei because the Mitzvah is merely to eat Matzah, and he did so. However, here we require "Zichron Teru'ah", and one who does not intend for the Mitzvah is Mis'asek (a Mishnah (32b) says that Mis'asek is not Yotzei). Rava teaches that this is not so.

Inference: Rava holds that Mitzvos do not require intent.

Question (Mishnah): If one was reading Parashas Shma in the Torah, and it was time for Kri'as Shma, he was Yotzei only if he had intent.

Inference: He was Yotzei only if he had intent to fulfill the Mitzvah. This shows that Mitzvos require intent!

Answer: No, he was proofreading; he was Yotzei if he had intent to read.

(Beraisa): One who hears a Shofar is not Yotzei unless he and the blower both intended.

R. Zeira told his attendant 'Have intent and blow for me'.

Inference: He holds that even if the blower wants to be Yotzei himself, he also needs to intend for the listener.

This does not apply to a Shali'ach Tzibur - since he has general intent for everyone, he need not intend for a particular listener;

(Mishnah): If one was outside a Beis ha'Keneses and heard the Shofar or Megilah, he was Yotzei only if he had intent.

Question (Abaye): If Mitzvos do not require intent, one who sleeps in a Sukah on Shemini Atzeres should be liable (for Bal Tosif (adding to a Mitzvah), even though he did not intend)!

Answer (Rava): I hold that one transgresses Bal Tosif only at the time for the Mitzvah.

32b - Mishnah: One is not Yotzei Teki'as Shofar if he was Mis'asek (intending to do something else). One who heard a Mis'asek blow was not Yotzei.

33b - Inference: (He was not Yotzei because he did not intend to sound the Shofar, e.g. he blew to clear out dirt or saliva.) Had he intended to play a song, he would be Yotzei!

This supports Rava, who says that one who intends for a song is Yotzei.

Rejection: Perhaps the Tana calls also this Mis'asek (because he did not intend for the Mitzvah).

Pesachim 114a: Tana'im argue about whether or not Mitzvos require intent:

(Beraisa #1): If one ate Maror without intent, he was Yotzei.

(Beraisa #2 - R. Yosi): Even though he ate already Chazeres (for Karpas), it is a Mitzvah to bring Chazeres.

Rishonim

The Rif (Rosh Hashanah 7b) brings the Beraisa that one is Yotzei only if he and the blower both intended for this, and R. Zeira's command to his attendant.

Question: The conclusion is like Rava, so why did the Rif bring R. Zeira?

Suggestion: R. Zeira does not argue with Rava. He requires intent of the blower and listener only for the sound, but he does not require intent for the Mitzvah.

Rejection (Ba'al ha'Ma'or): It is unreasonable that we do not require the primary intent, i.e. for the Mitzvah, but we require the blower and listener to intend for the sound! Also, we establish R. Zeira to hold like R. Yosi, and in Pesachim R. Yosi holds that Mitzvos require intent! The Halachah follows Chachamim who argue with R. Yosi; it does not follow R. Zeira.

Answer (Milchamos Hash-m): Really, R. Zeira requires intent for the Mitzvah. He argues with Rava.

Question: The Halachah should follow the later Amora, i.e. Rava!

Answer #1 (Milchamos Hash-m): Perhaps Rava himself requires intent for Mitzvos; he merely answered Abaye's question against Rabah (who said that Toke'a l'Shir is Yotzei, according to the Ramban's text). Since we exclude Rava, R. Zeira is Basra (Rabah preceded R. Zeira). Also, perhaps Rabah merely comments on the teaching of one who was forced to eat Matzah, but disagrees! In Pesachim, Reish Lakish and Rav Huna hold that Mitzvos require intent.

Answer #2 (Rosh, Rosh Hashanah 3:11): The Rif rules like R. Zeira because R. Yosi holds like him in Pesachim, and 'Nimuko Imo'. The Yerushalmi learns from the Mishnah of hearing the Shofar or Megilah from outside a Beis ha'Keneses that Mitzvos require intent. The Bavli also sought to learn this from a Beraisa. Poor rebuttals are given; the texts of the Mishnah and Beraisa connote otherwise.

Rebuttal (Or Zaru'a in Hagahos Ashri): R. Zeira is the only Amora who says that Mitzvos require intent. Several Amora'im and the Stam Gemara do not require intent. The Halachah follows Rav, who says that accidental immersion permits a Nidah to her husband (Chulin 31a).

Rambam (Hilchos Shofar 2:4): If one was practicing blowing a Shofar on Rosh Hashanah, he or one who heard from him was not Yotzei. One who hears a Shofar is not Yotzei unless he and the blower both intended that he be Yotzei.

Rambam (Hilchos Megilah 2:5): If one read the Megilah without intent he was not Yotzei.

Magid Mishnah: Perhaps all opinions would agree to this (only regarding Mitzvos involving an action some do not require intent, for the action is in place of intent - R. Yonah Berachos 6a DH Emnam).

Rebuttal (Lechem Mishneh): A Beraisa requires intent for Shofar and Megilah. The Gemara wanted to infer that Mitzvos require intent. It rejected this by saying that only intent to hear is required, i.e. it is like the opinion that does not require intent, and intent to be Yotzei is not needed even for Megilah!

Rosh (Rosh Hashanah 3:11): If one was outside a Beis ha'Keneses and heard the Shofar or Megilah, he was Yotzei only if he had intent. If one was forced to eat Matzah, he was Yotzei. Rava inferred that one who blows a Shofar for a song is Yotzei, for Mitzvos do not require intent. R. Zeira told his attendant 'Have intent and blow for me'. He holds that also the one blowing must have intent. We are stringent to require intent for Mitzvos; also the one blowing must intend for the listener.

Poskim

Shulchan Aruch (OC 60:4): Some say that Mitzvos do not require intent.

Source (Gra DH Yesh): They follow the majority who argue with R. Zeira and R. Yosi. Also, we conclude that accidental immersion is Metaher (Chulin 31a), and one fulfills the Mitzvah of Tefilin without intent (Eruvin 95b).

Ran (Rosh Hashanah 7b DH Garsinan): The argument is whether or not one needs intent to be Yotzei, but all require him to know what he is doing, e.g. he is eating Matzah and it is Pesach. If not, it would have been better to teach about this than about one who was forced!

Rebuttal (R. Yerucham, brought in Beis Yosef 475 DH Kosav ha'Rambam): We must say that the one who was forced to eat Matzah did not know that it was Matzah or that it was Pesach. One who does not want to be Yotzei is not Yotzei against his will!

Some say that Mitzvos require intent to fulfill the Mitzvah; this is the Halachah.

Source #1: Rosh Hashanah 28b expounds from "Zichron Teru'ah" that intent is required.

Source #2 (Yom Teru'ah ibid.): Abaye (Rosh Hashanah 28b) challenged Rava by saying that if Mitzvos do not require intent, one should be liable for Bal Tosif on Shemini Atzeres. If all agree that mid'Oraisa, intent is not required, all would agree that one transgresses Bal Tosif!

Magen Avraham (3): Mitzvos mid'Rabanan do not require intent.

Question: A Beraisa requires intent to hear the Megilah!

Answer #1 (Pri Megadim, Petichah Klalis 3:5): A Mitzvah from Nevi'im is considered like mid'Oraisa.

Answer #2 (Pri Megadim, Pesichah l'Birchos ha'Shachar DH Od Ra'isi Lehazkir): Perhaps the Magen Avraham holds that it is a Safek whom the Halachah follows, therefore we are stringent regarding Mitzvos mid'Oraisa (like the Ran, Rosh Hashanah 7b DH Lefichach), but we are lenient about Mitzvos mid'Rabanan.

Shulchan Aruch (OC 589:8): If one was practicing blowing a Shofar he was not Yotzei; also one who heard from him was not Yotzei.

Mishnah Berurah (15): If one came to the Beis ha'Keneses to hear the Shofar, even if he did not explicitly intend for the Mitzvah at the time of the Teki'os, he was Yotzei.

Eshel Avraham (5): If one is unsure whether or not he had intent, he must repeat the Mitzvah. We do not consider it a Sefek-Sefeka, even though many say that Mitzvos do not require intent.