Gemara

Mishnah: If one was reading Parashas Shema in the Torah, and it was time for Keri'as Shema, he was Yotzei only if he had intent.

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

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

Question: It says that he was reading!

Answer: He was proofreading.

Rashi (DH b'Korei): He did not even intend to read.

Tosfos (DH b'Korei): He did not read with the proper Nekudos (vowels), only in a way that will help him detect missing or extra letters.

13b (Beraisa - R. Meir): Intent is needed for the first verse of Shema.

Rava: The Halachah follows R. Meir.

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

Rava: This teaches that one who blows a Shofar for a song is Yotzei [if he heard the right notes. Some texts attribute this teaching to Rabah.]

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

Answer: One might have thought that he is Yotzei there because the Mitzvah is merely to eat Matzah, and he did so; but here "Zichron Teru'ah" is required, and one who does not intend for the Mitzvah is Mis'asek (engaged in another activity. A Mis'asek is not Yotzei - Mishnah 32b). Rava teaches that this is not so.

Inference: Rava holds that Mitzvos do not require intent.

Question (Mishnah): If one was reading Parashas Shema in the Torah... he was Yotzei only if he had intent.

Answer: He was Yotzei if he had intent to read.

Chulin 31a (Rav): If a Nidah immersed unwillingly, she is permitted to her husband, but she is forbidden to eat Terumah

R. Yochanan: She is forbidden even to her husband.

Question: What is the case of a Nidah immersing against her will?

Suggestion: Rachel forcibly put Leah in a Mikveh.

Rejection: If so, Rachel's intention would suffice, and Leah would be permitted even to eat Terumah!

Mishnah: A healthy woman can permit a deaf, insane, or blind woman to eat Terumah. (She checks her for Dam Nidah, and immerses her at the proper time.)

Answer (Rav Papa): Rather, she fell from a bridge, or entered the water to cool off, and fell in all the way.

Pesachim 114a (Mishnah): They bring to him Chazeres (lettuce). He dips and eats it before eating Matzah. Afterwards, they bring Matzah and Chazeres [for the Mitzvah of Maror]...

Reish Lakish: This teaches that Mitzvos require intent.

Question #1: Perhaps Mitzvos do not require intent, and he dips again to arouse the children!

Question #2 (Beraisa): If one ate Maror without intent, he was Yotzei.

Answer to both questions: Tana'im argue about this.

(Beraisa - R. Yosi): Even though he dipped (and ate) Chazeres (for Karpas), it is a Mitzvah to bring Chazeres, Charoses and two cooked foods.

Rav Huna: The first time he eats Chazeres he blesses Borei Peri ha'Adamah. The second time he blesses Al Achilas Maror.

Rav Chisda: He blesses Al Achilas Maror the first time.

The Halachah follows Rav Chisda.

Rishonim

Rif (Rosh Hashanah 7b): One is Yotzei Shofar only if he and the blower both intended for this.

Source #1 (Milchamos Hash-m): R. Zeira requires intent for Mitzvos, and he is later than the Amora'im who do not require intent. (We cannot prove from the Gemara what Rava himself held.) In Pesachim, Reish Lakish himself clearly holds that Mitzvos require intent, and also Rav Huna, who says that one blesses Al Achilas Maror the second time he eats it.

Source #2 (Rosh, Rosh Hashanah 3:11): In Pesachim R. Yosi holds like R. Zeira, and 'Nimuko Imo' (the Halachah follows R. Yosi because he had great reasons for his opinions). The Yerushalmi learns from a Mishnah of Shofar and Megilah that Mitzvos require intent. The Bavli also sought to learn this from a Beraisa; the rebuttals given are poor, for the text of the Mishnah and Beraisa connotes 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).

Milchamos Hash-m: When we asked from Keri'as Shema, we did not answer that 'he had intent' refers to the first verse. This shows that the opinion that does not require intent for Mitzvos holds that even though the first verse of Shema does not require intent. We hold that the first verse requires intent - this shows that Mitzvos require intent.

Question: If so, all of Shema should require intent!

Answer (Milchamos Hash-m): The Ikar of Shema is the first verse.

Rebuttal #1 (Rashba Berachos 13b DH Shema): If intent in the first verse is Me'akev, the Gemara should have inferred that Rava requires intent for Mitzvos! Rather, Rava (and R. Meir) discuss how much of Shema requires intent l'Chatchilah. Alternatively, even if the intent discussed is Me'akev, this refers to thinking about the meaning of the words. The argument about intent in Mitzvos is whether or not one must intend to be Yotzei the Mitzvah; if this is required, it is required for all of Shema.

Rebuttal #2 (Teshuvos Rashba 1:344): 'If one was reading in the Torah' connotes any Parashah, therefore we cannot say that it refers only to the first verse. Alternatively, the Gemara could have said that it refers only to the first verse (or Parashah); it chose to give a different answer.

Rambam (Hilchos Chametz u'Matzah 6:3): If one ate Matzah without intention, e.g. Nochrim forced him to eat, he was Yotzei.

Magid Mishnah: Some say that he was Yotzei only if he knew that it is Pesach and he is obligated to eat

Rambam (Hilchos Keri'as Shema 2:1): One is Yotzei Shema only if he had intent in the first verse.

Question: This is like the opinion that Mitzvos do not require intent, but regarding Shofar the Rambam requires intent (Hilchos Shofar 2:4)!

Answer #1 (Lechem Mishneh Hilchos Shofar 2:4): The Magid Mishnah says that the Rambam does not require intent for a Mitzvah done through an action. Perhaps saying Keri'as Shema is considered an action, therefore intent is not required.

Objection #1: The Magid Mishnah would not consider speech to be an action.

Source (R. Yonah Berachos 6a DH Emnam): An action is in place of intent. Speech depends on the heart, so if one spoke without intent he did not do anything that can be considered a Mitzvah

Objection #2: Also, the Magid Mishnah suggested that the Rambam requires intent for Matzah; if so, he must have another reason to distinguish Shema from Shofar and Matzah!

Answer #2 (Lechem Mishneh): The Rambam holds like the Milchamos Hash-m, who says that Rava requires intent only in the first verse because he holds that Mitzvos require intent; in Rosh Hashanah, Rava answered on behalf of the opposing opinion.

Rosh (Berachos 2:13): The Halachah follows R. Meir, who requires intent for the first verse of Shema.

Ma'adanei Yom Tov (8): This refers to intent for the meaning of the words; he should not think about other things, in order to accept Ol Malchus Shomayim. Regarding intent to fulfill the Mitzvah, it suffices to have intent when he begins to read.

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, the conclusion is that accidental immersion is Metaher (Chulin 31a), and that 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!

Beis Yosef (589, DH Lefichach): The Ro'oh learns from here that one is Yotzei against his will. This is a minority opinion, and we are not concerned for it.

Shulchan Aruch (ibid.): Some say that Mitzvos require intent to fulfill the Mitzvah. This is the Halachah.

Source (in addition to the Rif's sources above (b:1:i,ii,iii) - Tosfos Pesachim 115a DH Maskif). If Mitzvos did not need intent, there would be no need for the Gemara to say that the Halachah follows Rav Chisda. If one was Yotzei Maror the first time, blessing Al Achilas Maror the second time is a Berachah l'Vatalah!

Eshel Avraham (3): It is not clear whether or not the opinion that requires intent requires it mid'Oraisa.

Bi'ur Halachah (DH v'Yesh Omrim she'Tzrichos): The Rashbam and the other Acharonim say that the opinion that requires intent requires it mid'Oraisa.

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

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

Mishnah Berurah (7,8): This discusses intent to fulfill the Mitzvah. Intent for the meaning of the words is l'Chatchilah in every verbal Mitzvah. It is Me'akev only in the first verse of Shema and the first Berachah of Shemoneh Esre. If one is in doubt whether or not he intended, this is a Safek mid'Oraisa, so he must repeat Shema. However, he should not repeat the Berachos.

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.

Mishnah Berurah (9). According to the Magen Avraham, one would not repeat any Berachah said without intent except for Birkas ha'Mazon, since all other Berachos are mid'Rabanan. However, the Gra does not distinguish mid'Oraisa from mid'Rabanan. It appears that also the Shulchan Aruch does not. The Chayei Adam says that the argument is only when it is reasonable to say that he had a different intent, e.g. he was reading the Torah or practicing to blow. However, if one did a Mitzvah the usual way, e.g. he said Shema during Shacharis or took the Lulav (at the time of Hallel), even if he did not explicitly intend for the Mitzvah, it is clear that he did so to be Yotzei.

Bach (625 DH ba'Sukos): When the Torah specifies the reason for a Mitzvah, e.g. wearing Tzitzis to remember the Mitzvos, Sukah to remember Yetzi'as Miztrayim, or Tefilin so that remembering Yetzi'as Miztrayim will make Torah fluent in our mouths, this intent is needed to properly fulfill the Mitzvah.

Kaf ha'Chayim (10,11): A Mitzvah without intent is like a body without a Neshamah. Before any Mitzvah or prayer one should say 'l'Shem Yichud...', to unite Hash-m's name and the Shechinah. One must be extremely careful not to say this without intent.

Shulchan Aruch (5): If one did not have intent in the first verse of Shema; he was not Yotzei. If he intended in the first verse, he is Yotzei even if in the rest of Shema he was reading in the Torah or proofreading.

Mishnah Berurah (11-12): This refers to intent for the meaning of the words. Most say that intent to be Yotzei is necessary for all of Shema.

Shulchan Aruch (63:4): Even the opinion that does not require intent for Mitzvos agrees that if one read the first verse of Shema without intent he was not Yotzei and must repeat it.

Source (Gra DH v'Afilu): The Halachah follows Rava (Berachos 13b), the latest Amora in the argument. Even though he does not require intent for Mitzvos, he requires intent for the first verse of Shema.

Magen Avraham (5,6): 'Intent in the first verse' includes Baruch Shem Kavod.... When one must repeat the first verse, he must repeat the entire Parashah.

Note: The Gemara answered that intent to read in the Torah suffices to be Yotzei. Baruch Shem... is not written in the Torah! We must say that the discussion is whether or not he was Yotzei the first verse. However, this is unlike the Ramban and Rashba cited above (b:1:vii).