Gemara

29a (Rav Kahana): If Levi hid witnesses in back of curtains around the bed, and David admitted that he owed him, but when Levi asked 'may the waking and sleeping be witnesses?', David said 'no', he is exempt.

38a (Beraisa - R. Meir): People are different from each other in voice and appearance due to Ervah (lest men have Bi'ah with others' wives).

67a (Beraisa): Mesis (one who entices to serve idolatry) is the only capital transgression in which witnesses hide in order to trap someone. We light a candle an inner room, and witnesses are in the outer room. They can see and hear the Mesis, but he cannot see them.

Gitin 23a (Rav Sheshes): A Shali'ach to give a Get cannot be a Suma (blind), for he doesn't know who gave it to him, and to whom he gives it.

Objection (Rav Yosef): If a Suma cannot recognize people, why may he have Bi'ah with his wife (perhaps it is another's wife)? Why may anyone have Bi'ah at night (when he cannot see)? Rather, he has Tevi'as Kala (recognizes his wife's voice). Similarly, he could recognize the giver and receiver of a Get!

Chulin 96a (Rava): I used to think that a Siman is better than Tevi'as Ayin, for we return a lost object to anyone based on a Siman, but through Tevi'as Ayin, we return only to a Chacham. I retract due to teachings that show that Tevi'as Ayin is no worse (it permitted meat and Techeles).

Support: Tevi'as Kala permits a Suma to his wife. Likewise, we may rely on Tevi'as Ayin! Also, R. Yitzchak taught that testimony of Simanim on one's body and clothing is not enough to kill, but witnesses' Tevi'as Ayin is enough!

Yevamos 122a (Mishnah): We may testify based on a voice, even if no body was seen. A case occurred in which a man announced 'I, Ploni, am dying.' People went and did not find anyone. His wife was permitted to remarry.

Nidah 12a (Beraisa): If a man returned home... and his wife was Tehorah when he left, he may be with her, whether she is sleeping or awake.

Rishonim

Rif (Chulin 34a): The Gemara proves that Tevi'as Ayin is better than Simanim. If not, a Suma, or anyone at night, would be forbidden to have Bi'ah!

Rambam (Hilchos Edus 9:12): A Suma is Pasul for testimony, even if he recognizes voices and knows people - "v'Hu Ed Oh Ro'oh Oh Yoda."

Rambam (Hilchos Ishus 24:19): If a woman mistakenly had Bi'ah with another man b'Shogeg or Ones, she is permitted to her husband.

Yad ha'Melech (DH v'Ra'isi): The Kesef Mishneh explains that she mistakenly had Bi'ah with another man. This is not Shogeg. She needed to verify that it is her husband. This is rebellion and she is forbidden to him. A blind man, or anyone at night, may have Bi'ah only due to Tevi'as Kala. One may not rely on estimation! Rather, she received a Get and did not know that it is Pasul, or witnesses falsely testified that her husband died.

Ran (Gitin 11b DH Ela): Tosfos says that perhaps only a Suma must recognize the couple to be a Shali'ach to divorce, lest people trick him. This is primary. Only a Suma is Pasul, for he must rely on others. A Pike'ach (seeing person) can be a Shali'ach even if he does not recognize them (Gitin 29b). We concluded that a Suma can have Tevi'as Kala. He is Pasul only for a Get from Chutz la'Aretz. (The Shali'ach must see it written.)

Question: If one heard Ploni curse or excommunicate Almoni, but did not see, may he testify about this? In Yevamos, we require seeing a human form with a shadow of a shadow. We should require the same in all cases. Or, perhaps this is only for a Get, which is Isur. Elsewhere, we allow witnesses to write even if they do not recognize (so one must bring a proof to collect)!

Answer (Ri mi'Gash 149): If one recognizes his friend's words, he may testify even if he did not see him. Tevi'as Kala permits Bi'ah to a Suma or at night. We permit a widow based on a voice, only if they recognized it, after seeing a shadow of a shadow, so they know that it is not a Shed. We rely on Tevi'as Kala not only for Agunah (one who lacks proper testimony to remarry), rather, also for Bi'ah. Witnesses in back of the fence are valid, if the borrower accepts that anyone may testify about his admission, even though they only hear his voice. Do not say that they saw him, but he did not see them. We do not put into a teaching what is not explicit, unless there is a proof.

Ramah (67a Sof DH Piska): In Gitin we say that we rely on Tevi'as Kala (to give a Get). That is not testimony. Witnesses must see the person and hear his voice. We permit a Suma to his wife through Tevi'as Kala, even though he is Pasul for testimony! Also, in Yevamos, we require seeing a shadow of a shadow. We do not say that Tevi'as Kala removes concern for a Shed!

Note: Perhaps Shedim are good at mimicking voices, so Tevi'as Kala does not help for this! Ashmedai (the king of the Shedim) made people think that he was Shlomo (Gitin 68b)! Perhaps he can mimic people better than Stam Shedim. If not, even if we see the husband we should be concerned!

Poskim

Shulchan Aruch (EH 132:4): If a Shali'ach lost a Get and found it later, if he has Tevi'as Ayin on it, the Get is Kosher.

Beis Shmuel (6): Tosfos says that even an Am ha'Aretz has Tevi'as Ayin, just we are concerned lest he lies. Here we believe anyone with a Migo (he himself found it), and a Chacham even without a Migo.

Shulchan Aruch (142:11): A Suma cannot bring a Get from Chutz la'Aretz, for he cannot say 'it was written and signed in front of me.' If it was written in front of a Pike'ach, and later he became blind, he can make the declaration and give it to her. He must recognize her voice. If not, he cannot give it. Only a Pike'ach may rely on others who say that it is the sender's wife.

Beis Shmuel (18): The Magid Mishneh (Hilchos Gerushin 3:9) says that nowadays, not even a Chacham is believed through Tevi'as Ayin. It is clear from Chulin that Tevi'as Ayin and Tevi'as Kala are the same. Why does Tevi'as Kala help here? Do not say that we test if he recognizes her voice. If so, we could rely on this even if he was always blind! Perhaps here we rely on his Tevi'as Kala because we know that she is the wife of the one who sent the Get. If so, this is only if we know this. If he tells us that she is the sender's wife, we may not rely on him.

Yam Shel Shlomo (Chulin 7:26): Why is a deaf person, even if he can see, permitted to his wife in the dark? I say that Tevi'as Kala is needed only when she is not found in her normal place. If she is in her room, in her bed, surely we are not concerned lest it is someone else. One need not speak with his wife before Bi'ah for to verify that it is her, rather, to appease her. One need not speak at all if he sees that she desires him. One who returns home may have Bi'ah with her without waking her. It is difficult to say that this is after he lit a lamp and saw her.

Ya'avetz (Chulin 96a): If so, how did the Gemara prove that we rely on Tevi'as Kala? Perhaps one's wife is permitted only in the house!

Mishneh Hilchos (4:185): Avos d'R. Noson (1:4) explains people differ in voice, lest Ploni leave his house and Almoni enter and have Bi'ah with Ploni's wife. People have different faces, lest men not recognize their wives and vice-versa. I.e. a man need not verify that the woman in his house is his wife. She knows that he is her husband through Tevi'as Kala. Outside his house, he recognizes her appearance, for one may not speak with a woman in public, even with his wife. (Note: Avos 1:5 says not to speak much with her.) Regarding this the Gemara teaches that if he is blind or it is dark, he relies on Tevi'as Kala.

Chasam Sofer (Gitin 23a DH ha'Kol): The Kesef Mishneh says that the Rambam learns from Gitin 23a that a Suma is Pasul mid'Oraisa for testimony. This is difficult. Seemingly, one could know testimony without seeing. A Gezeras ha'Kasuv disqualifies. We should disqualify only a Suma, but a Pike'ach can testify based on Tevi'as Kala. However, Rashi (Sanhedrin 67a) holds that it is not a Gezeras ha'Kasuv. If a Pike'ach did not see the event, the defendant could say 'it was not me.' This is difficult, for we concluded that Tevi'as Kala is full recognition! Rather, this must refer to Tevi'as Kala with feeling. The Ramban (Bereishis 27:12) says that some people can mimic others' voices; this is why Yitzchak felt Yakov. He did not rely on voice alone. Perhaps one recognizes primarily through feeling. This explains why one who returns from a trip may he have Bi'ah with his wife at night without talking with her?

Note: If he must suspect lest she is another's wife, he may not touch her to feel her (or be secluded with her)! How can a man with only one wife know that he would feel the difference if it were someone else? He does not feel other women!

Achiezer (3:15 DH v'Hinei): A case occurred in which Ploni saw a lying man say 'help! I'm dying!' He recognized his voice, but not his face. Two witnesses saw him stabbed in war, and he is not found, so mid'Oraisa a majority permits him. In such a case, we rely on one witness (even in war). A Chacham asked from the Ramah, who derives that Tevi'as Kala does not help for Agunah!! However, the Ri mi'Gash (149) says that Tevi'as Kala helps to force one to pay, and also for Davar sheb'Ervah. The Rashba (1179) connotes that Tevi'as Kala helps for Kidushin. (He disqualified a Kidushin only because a witness was unsure who took the money, and they did not know that she took it l'Shem Kidushin.)

Tzitz Eliezer (10:47:3): Beis Din Yitzchak (EH 2:53) allowed a woman about to fall to a minor Yavam (she would be an Agunah until he matures) to appoint over the telephone a Shali'ach to receive her Get. Even though it is not proper Tevi'as Kala, the Shali'ach can recognize her voice. Two witnesses must see her command on the phone, and two must hear with the Shali'ach. A husband cannot command about a Get in this way, for the husband's Shali'ach must hear his voice.