Gemara

24b (Beraisa): If one was sleeping in [only] his Talis and it is too cold to stick out his head (to say Shema), he presses his Talis on his neck (to separate between his eyes and his Ervah);

Some say, he presses it on his heart.

Question: According to the first Tana, Libo Ro'eh Es ha'Ervah! (His heart 'sees' his Ervah, i.e. there is no separation between them.)

Answer: He permits [saying Shema when] Libo Ro'eh Es ha'Ervah.

25b (Mishnah): If not, he covers himself in the water and says Shema.

Question: Libo Ro'eh Es ha'Ervah!

Answer (R. Elazar or Rav Acha): The case is, the water is cloudy. It is like a covering of earth.

(Beraisa): One may sit in clear water up to his neck and say Shema;

Some say, he must make the water cloudy with his feet.

Question: According to the first Tana, his heart sees his Ervah!

Answer: He permits Libo Ro'eh Es ha'Ervah.

Question: According to the first Tana, his heel sees his Ervah!

Answer: He permits this;

(Abaye): Akevo Ro'eh Es ha'Ervah is forbidden;

(Rava): It is permitted. People are not angels.

Shabbos 150a (R. Yochanan): "V'Daber Davar" forbids speech (about Melachah). Thoughts are permitted.

Question (Rav Acha bar Rav Huna): R. Yochanan forbids thinking about Torah in a bathhouse or bathroom, due to "v'Hayah Machanecha Kodesh". He should expound "v'Lo Yir'eh Vecha Ervas Davar" to forbid speech and permit thoughts!

Answer: "Ervas Davar" teaches Rav Yehudah's law:

(Rav Yehudah): One may not say Shema facing the Ervah of a Nochri.

It is forbidden facing even a Yisrael's Ervah. One might have thought that a Nochri's Ervah is like an animal's. He teaches that this is not so.

Rishonim

Rif (15a): The Halachah is, Libo Ro'eh Es ha'Ervah is forbidden.

Rambam (Hilchos Keri'as Shema 3:17): If one was sleeping in his Talis, he presses it below his heart and reads Shema. He may not press it on his neck and read, for Libo Ro'eh Es ha'Ervah. It is as if he reads without a belt.

Rosh (3:36): The Halachah forbids Libo Ro'eh Es ha'Ervah. Also R. Chananel says so, since the Gemara asked Stam 'Libo Ro'eh Es ha'Ervah'! R. Shemayah said in the name of Rashi that he found in Teshuvas ha'Bavli that Libo Ro'eh Es ha'Ervah is permitted. The first Tana is like a majority against an individual. However, Abaye and Rava argue about whether we forbid when his heel sees his Ervah. This connotes both forbid Libo Ro'eh Es ha'Ervah. Shmuel (Sukah 10b) permits one sleeping (naked) in a canopy to stick out his head and read Keri'as Shema, for he permits Libo Ro'eh Es ha'Ervah. However, we follow Abaye and Rava, who were Basra, against Shmuel. When R. Tam would bathe and he wanted to drink, he would use a garment to cover below his heart (before blessing), lest Libo Ro'eh Es ha'Ervah.

Rashi (25b DH v'Harei): The Gemara is thinking that any Ever that normally does not see the Ervah, if it sees the heart when he reads Torah, we apply "v'Lo Yir'eh Vecha Ervas Davar."

Gra (DH Hayah): Rashi permits Libo Ro'eh Es ha'Ervah, based on how he learned the Sugya. Rashi was bothered. We said that the Tana permits Libo Ro'eh Es ha'Ervah. What was the question from the heel? Rather, the Isur is for limbs that normally do not see the Ervah. The heel is less prone to see the Ervah than the heart is (so there is more reason to forbid it. Amora'im argue about the heel, but all permit Libo Ro'eh Es ha'Ervah.)

Rashba (25b DH veha'Lo, citing the Ra'avad): When his face is out of the water, the Gemara was concerned only for Libo Ro'eh Es ha'Ervah, and answered that the water is cloudy. We were not concerned for his eyes seeing his Ervah, since his eyes are out of the water and look outside. They do not see his Ervah.

Beis Yosef (u'Mah she'Chasav Rabeinu): Also R. Yonah says so. I.e. it is permitted even though his eyes could see his Ervah (if he would look down), since he does not look at it.

Poskim

Shulchan Aruch (OC 74:1): If one was sleeping in only a Talis, he must press his Talis on his heart and then he may read, for Libo Ro'eh Es ha'Ervah is forbidden.

Beis Yosef (DH Hayah): The Rif, Rambam, Rosh and other Rishonim forbid Libo Ro'eh Es ha'Ervah. Surely we follow them. Terumas ha'Deshen (10, citing Or Zaru'a) says that one must separate with a garment. It does not suffice to cover with one's hands.

Mishnah Berurah (1): The Mechaber cited the words of the Gemara. The same applies to one who is in a robe without pants. He may not say Divrei Torah or any Berachah until he presses his robe at the heart or below to separate his heart from his Ervah.

Mishnah Berurah (3): If there is a separation it is permitted even if his heart is uncovered.

Mishnah Berurah (4): Chachamim were stringent about the heart because a verse discusses the heart seeing - "v'Libi Ra'ah Chachmah Harbeh." Even if a garment covers his heart to separate it from his Ervah, also his Ervah must be covered. Many Acharonim forbid mid'Oraisa if it is exposed, even if there is a Talis on his head and he cannot see it. It does not say 'you will not see Ervas Davar", rather, "Ervas Davar will not be seen in you." I.e. it must be covered so even others cannot see it. If one was naked in a house and stuck his head outside the window, even if he put a garment over his heart, he may not read because his Ervah is exposed. If one was sleeping in a canopy less than 10 Tefachim high and stuck his head outside the window, and made a separation between his heart and his Ervah, it is permitted. Even though his Ervah is exposed in the canopy, it is as if it is covered with a garment. Because the canopy is less than 10 Tefachim tall, it is considered a covering, like a garment. It is unlike a house, which is a separate Reshus. We cannot call a house a covering. Similarly, if excrement is in the house within four Amos of him, and he stuck his head outside the window, he may not read Keri'as Shema, for his head gets the law of his body. If he stuck his head into a house with excrement, all forbid, for the excrement is in front of his eyes. The Rashba forbids even if his head is outside the window, since he faces the excrement.

Mishnah Berurah (5): Derech ha'Chayim and the Eshel Avraham (71:5) say that b'Di'eved, if one said Shema or prayed, he repeats it, including the Berachos of Shema, for Chachamim enacted like this. It is a Safek regarding other Berachos, so we are lenient about Safek Berachos. Chayei Adam is lenient also about Tefilah regarding a mid'Rabanan matter. Perhaps one should follow him for this matter. We can add the opinion of Rashi (who permits Libo Ro'eh Es ha'Ervah).

Bi'ur Halachah (DH Hayah): When his heart does not see his Ervah, it is permitted even if his head is under the same garment. This is clear from 24b.

Bi'ur Halachah (DH Tzarich): Divrei Chamudos (3:114) and Eliyahu Rabah (1) say that if a garment covers his heart but not his Ervah, this permits even if it is not taut on his heart.

Bi'ur Halachah (DH Mishum): If his head and heart are both outside of the canopy, it is forbidden, for it is as if he is totally outside.

Rema: Likewise, if his heart sees another's Ervah, it is forbidden.

Gra (DH v'Hu): We learn from Rav Yehudah, who forbids facing even the Ervah of a Nochri.

Mishnah Berurah (6): If Reuven and Shimon are under one garment facing each other, even if Reuven has a separation between his heart and his Ervah, it is forbidden because his heart sees Shimon's Ervah. The same applies if they are in water, even if Reuven's eyes are out of the water.

Kaf ha'Chayim (7): The same applies to Ervah of a minor to whom Bi'ah applies (a boy above nine years or a girl above three years). A Torah Lav forbids when one's eyes see his Ervah or another's Ervah - "v'Lo Yir'eh Vecha Ervas Davar." This is a real Drashah. However, one may think Torah at the time. This is clear from Shabbos 150a. Shome'a k'Oneh is forbidden, for this is like speaking. The Aseh "v'Hayah Machanecha Kodesh" applies only to excrement. Therefore, for Ervah it suffices to turn the head away. The Rashba and Rosh argue about whether [the end of this verse,] v'Lo Yir'eh Vecha Ervas Davar applies also to excrement. Libo Ro'eh Es ha'Ervah, or another's Ervah, is mid'Rabanan. Poskim argue about whether the Isur when one's Ervah is exposed is mid'Oraisa or mid'Rabanan. If one read, he reads again due to a Safek about a Torah law, but not for a Safek mid'Rabanan.