Gemara

(Mishnah): If a house collapsed on top of Chametz, it is as if Bi'ur was done.

(Rav Chisda): One must be Mevatel it in his heart.

Shabbos 150a (R. Yochanan): R. Yehoshua ben Korchah expounds "mi'Mtzo Cheftzecha v'Daber Davar" - Dibur (talking, i.e. about Melachah) is forbidden, but Hirhur (thoughts) are permitted.

Inference (Rav Acha bar Rav Huna): Dibur is unlike Hirhur.

Berachos 20b (Mishnah): A Ba'al Keri must think in his mind Keri'as Shema, but not [even think] the blessings before and after it.

(Ravina): Our Mishnah teaches that Hirhur k'Dibur. If it were not, there is no reason to have Hirhur!

(Rav Chisda): Hirhur is not like Dibur. If it were like Dibur, he should say the words!

Rishonim

Rosh (Megilah 3:1): One who receives an Aliyah must read quietly and accurately with the Shali'ach Tzibur. If not, his Berachos are l'Vatalah.

Rosh (Pesachim 1:10): Why do we bless Al Bi'ur Chametz? We should bless Al Bedikas Chametz"! They enacted to bless Al Bi'ur Chametz, for right after Bedikah one does Bitul, which is Bi'ur of the Chametz he does not know about. In the morning, in the fifth hour, he eradicates it from the house. The Berachah was enacted on Bedikah, which is the beginning of Bi'ur.

Poskim

Shulchan Aruch (OC 432:1): Before beginning to check, one blesses "... Al Bi'ur Chametz."

Beis Yosef (DH u'Mah): Since the Berachah is for Bi'ur, why do we bless before Bedikah? We should bless close to the time of Bi'ur, after the Bedikah! The Tur answers that Bedikah is the beginning of Bi'ur. Perhaps the Tur comes to teach why we bless on Bi'ur, and not on Bedikah, like the Rosh says. We do not bless on Bitul Chametz, for it is Devar sheb'Lev. We do not bless on Devarim sheb'Lev. Also, Bitul is included in Bi'ur, but Bi'ur is not included in Bitul. We do not bless on Bedikah, for if one only did Bedikah but not Bi'ur, he did not do anything.

Taz (1): The Beis Yosef says that we do not bless on intent.

Sha'arei Teshuvah (1): Leket ha'Kemach (Hilchos Pesach veha'Sefirah DH Nusach) brings from the Radvaz (1359/4:288) that the text of the Berachah is on the purpose of the Mitzvah, which is Bi'ur, and not on Bitul, which is thought in the heart.

Shulchan Aruch (4): One who thinks Divrei Torah need not bless.

Gra (DH Hirhur and DH v'Hu): Also Shabbos 150a shows that Hirhur Lav k'Dibur. Here is difficult, for he blesses on the Mitzvah. Is there no Mitzvah without Dibur?! It says "v'Hagisa (Bo Yomam va'Laylah)", i.e. in the heart - "v'Hegyon Libi"!

Damesek Eliezer: I would answer that we do not find a Berachah for mere Hirhur. If one thinks about love and fear of Hash-m, should he bless?! The Gra (8:2) gives general rules of Berachos. One is that we bless only on actions.

Machazik Berachah (6): Chachamim enacted Berachos only on matters that have an action, to exclude matters like fear and love of Hash-m, which are in the heart, and they apply at all times. If one took chicks under their mother, if he does not take the mother, he fulfills the Mitzvah passively. Therefore, even if he took the mother and sent her, he does not bless. The Ra'avad and Rashba bring from Ri ben Palat that we do not bless. Teshuvas ha'Rashba (283) says that some say to bless, but the Rashba wavered. In Sefer ha'Pardes, the Rashba's son says that the Rashba said that the general rules of which Berachos we bless on are folly. We bless only on the Mitzvos for which Chachamim enacted Berachos.

Hilchos Ketanos (2:159): One must bless for hearing Divrei Torah, since Shome'a k'Oneh. This is like hearing the Shofar or Megilah. The Agur said that one does not bless on Hirhur b'Torah, for it is not k'Dibur. He did not say so about one who hears. Also, perhaps the Agur's law is not reliable. We forbid Hirhur near a foul odor. Also, we bless La'asok b'Divrei Torah. Hirhur in Torah is engaging in Torah! Keri'as Shema is different, for it says "v'Dibarta Bam." Even so, if one read in his heart due to Ones, he was Yotzei. I say that the Halachah follows Ravina. The Agur and Maharil forbid Hirhur on Tish'ah b'Av. They do not distinguish that there, it is due to Simchah.

Shulchan Aruch (141:2): One who receives an Aliyah may not read loudly. However, he must read quietly with the Shali'ach Tzibur. If not, his Berachos are l'Vatalah.

Pri Chodosh (2): Why don't we permit due to Shome'a k'Oneh, like for Megilah (692:1) and Shofar (585:3)?

Taz (3): Why must the Oleh read? The Mordechai (Megilah 832) brings from the Tosefta (Megilah 2:2) and Yerushalmi a case in which R. Meir read, and someone else blessed, due to Shome'a k'Oneh! This supports what the Beis Yosef brought from the Agudah and Nimukei Yosef, that a blind person may bless and someone else will read. The Beis Yosef rules like those who forbid.

Machazik Berachah (47:4): Even if Shome'a k'Oneh, this does not show that one must bless. The Rosh, Tur and Shulchan Aruch say that if one who receives an Aliyah does not read, his Berachos are l'Vatalah. We do not say that Shome'a k'Oneh! Zekan Aharon (60) says that due to Shome'a k'Oneh, it is as if one answered to Hallel, Kedushah or Kaddish, but one may not bless based on what someone else will do, e.g. Ploni will bless with intent that Almoni will read the Megilah for him. Hilchos Ketanos can argue with Zekan Aharon, but not with the Rosh, Tur and Shulchan Aruch! He can say that the enactment was for the Oleh to bless on his own reading. Therefore, Shome'a k'Oneh does not help. The Taz and Pri Chodosh asked why he must read, and Shome'a k'Oneh does not suffice. She'alas Ya'avetz (brought below) gave a weak answer.

Sha'arei Teshuvah (47:3): Hilchos Ketanos (2:159) obligates blessing for hearing Divrei Torah. Since Shome'a k'Oneh, this is more than mere Hirhur.

Mishnah Berurah (47:6): One may give a ruling between litigants, not in the way of learning. To learn a law in a Sefer without the reason, one must bless. This is no less than writing.

Machazik Berachah (489:4): If Bein ha'Shemashos, one wrote in a letter [e.g. at the top] 'on the 17th day of Sefirah', may he bless afterwards? I prove from Sefer Chasidim (398) that writing is not like Dibur. Someone in Cherem needs permission from the Tzibur to write to someone. They can reveal that they did not intend to forbid this. If writing were like Dibur, this is like speaking, and permission would not help unless they permitted the entire Cherem.

She'alas Ya'avetz (2:140): If one read Keri'as Shema silently, he was Yotzei. Do not prove from here that Hirhur k'Dibur. He was Yotzei only b'Di'eved [and Tosfos rules that he was not Yotzei]. This is for Keri'as Shema, for it says "Al Levavchem", and for Divrei Torah, since the simple meaning of the verse discusses Divrei Torah. Investigation and thought about Torah is also a Mitzvah - "v'Hagisah Bo." Normally, Hirhur is unlike Dibur I proved this from Megilah 18b. If one was writing a Megilah, he is Yotzei Kri'as ha'Megilah only if he said every verse.

R. Akiva Eiger (Teshuvah 1:29): Be'er Heitev brings from Teshuvos Matzah Shemurah that one who wrote the day of the Omer was not Yotzei. Shev Yakov (OC 48) brought from Re'em and others that one who wrote a Shevu'ah is not obligated, until he says it. Terumas ha'Deshen says that one who wrote 'I said a Shevu'ah' cannot deny it. This implies that writing a Shevu'ah does not obligate. He proved from Megilah 18 that Hirhur Lav k'Dibur. Do not say that Shevu'ah is different, for it says "Levatei bi'Sefasayim", for he learned from Megilah. The verses about Sefiras ha'Omer do not connote that one must say it any more than for the obligation of a Zavah to count seven clean days. No Posek said that she must count with her mouth. The Beis Yosef said that Chachamim enacted to bless on Bedikas Chametz, which is mid'Rabanan, and not on Bitul, which is mid'Oraisa, for it is in the heart, and we do not bless on Devarim sheb'Lev. Therefore, at least mid'Rabanan, one must say which day of the Omer it is

R. Akiva Eiger: The questioner asked that we bless on separating Terumah, even though one can separate through Hirhur. I answer based on Tosfos (Chulin 6b DH v'Hitir), who permits this only for Demai. This is why a naked person may tithe Demai, but not Vadai Tevel. Also, mid'Oraisa Hirhur suffices, but mid'Rabanan Dibur is required.

Beis She'arim (OC 291): Ha'Motzi is not considered Birkas ha'Mitzvah, even on Shabbos. This is because it is the same Berachah we say during the week. Only the intent is different, that he eats for the Mitzvah. The Beis Yosef taught that we do not bless on Devarim sheb'Lev.