Gemara

(Beraisa - R. Eliezer b'Rebbi Tzadok): A certain woman used to "miscarry" red peels (they would fall from her Ervah). My father asked (the Sanhedrin in exile) in Yavneh, and they asked doctors. They said that the scabs are from a wound inside.

Yoma 84b (Beraisa): We do not do these matters (of Piku'ach Nefesh) through Nochrim or Kusim, and we do not say 'let us do them based on women or based on Kusim', but they join to another opinion.

Avodah Zarah 27a (Mishnah): One may use a Nochri for monetary cures, but not for Nefashos (mortal cures).

Question: What is the difference between these?

Suggestion: A monetary cure is when the Nochri is paid.

Rejection: If so, the Mishnah should say 'one may pay a Nochri to cure him, but he may not be cured by a Nochri for free'!

Answer #1: Nefashos is when there is danger.

Question: Rav Yehudah forbids using a Nochri to heal the wound after bloodletting (which is not dangerous)!

Answer #2: Monetary cures are of the Yisrael's animal. Nefashos is of the Yisrael himself.

(Rav Chisda): If a Nochri doctor says 'this drug will help you, but this one will harm you', one may rely on him.

The Nochri assumes that the patient will ask others. He fears ruining his reputation.

Rishonim

Rambam (Hilchos Rotze'ach 12:10): One may ask a Nochri doctor, and he says 'medicine Ploni is good for you, and do such and such.' However, one may not take it from him.

Rosh (Yoma 8:4): The Sifri, Bahag and Tosefta say 'we do not do these (matters of Piku'ach Nefesh) through women or based on Nochrim. The Riva asked, why can't we do them through women who are expert in mortal matters? Regarding a Yoledes we are Mechalel Shabbos based on women's opinion! The Riva's text says 'we do not do these based on women or based on Nochrim, but they join with another opinion. I.e. we do not say that they are not believed. Rather, they even join to contradict men. The word 'but' does not fit so well. The Rif's text says 'we do not say 'let us do these through women or based on Nochrim', because they join with another opinion.' I.e. the Nochri will say 'they are not concerned if we do Aveiros', and also women will say so. 'They join with another opinion' means that they will err to say that we are not concerned if they sin, and there will be danger (since will refrain from what is needed to save the life).

Ran (Yoma 4b DH Ein): Rashi explains that we are not Mechalel Shabbos based on what women or Kusim say, but if two (Yisre'elim) say that we must be Mechalel Shabbos and three say that we need not, a woman or Kusi joins with the two and makes it a Safek, and we are lenient about Safek Nefashos. Others challenged this, for we rely on midwives to say when one must be Mechalel Shabbos for a woman giving birth. This is not difficult, for women are more expert about this than men are. However, clearly if a woman says 'he needs' and the men are unsure, this is a Safek and we are lenient.

Question (Teshuvas ha'Rosh 2:18): Why did Chachamim said that the peels must be cast into water? Even if we find that they are blood, the doctors said that they are from a wound, so she is Tehorah!

Answer (Maharam Lublin Teshuvah 111): Chachamim did so to check whether the doctors were correct.

Question (Chacham Tzvi 46): If a woman saw blood and doctors say that she has a wound, she is Tehorah. We need not check, even though there is blood. All the more so we should rely on doctors regarding peels! Also, since we do not rely on the doctors, why were their words mentioned?

Answer (Sidrei Taharah 188:5): Maharam Lublin connotes that we do not rely on doctors for Isurim. However, perhaps this is only when we can check. The Chacham Tzvi's question is only according to the Ba'al ha'Ma'or, who holds that any dry blood that does not dissolve is Tahor. The Ra'avad holds that this is only if the blood has the form of a creation. If so, the doctors' words (that they are scabs of a wound) were needed. Without them, even soaking could not be Metaher her. The Rosh asked, for he holds like the Ba'al ha'Ma'or. Some say that the doctors said to cast it into water. This is difficult. Shev Yakov (1:37) answered that we believe doctors when they have a proof that there is a wound. Here, they said so merely because it is uncommon for a woman to miscarry such peels.

Maharik (159): Nidah 22b connotes that they relied on doctors even when there is concern for Kares. One could reject this. Perhaps the situation itself suggested this (that it is not Dam Nidah), for it is not normal to see Dam Nidah that way, but on the other hand, they saw red hairs and peels. Cases regularly occur in which we rely on Nochri doctors to feed Cholim on Yom Kipur.

Poskim

Shulchan Aruch (OC 328:10): Some say that we do not require an expert to say that a Choleh is dangerously sick to permit Chilul Shabbos. All people are somewhat expert, and we are lenient about Safek mortal danger.

Mishnah Berurah (25): We rely on doctors, even if they are Nochrim.

Mishnah Berurah (27): The person must say that he knows the sickness.

Rema: Some say that this is only Yisre'elim, but we do not consider Stam Nochrim who are not doctors to be experts.

Gra (DH v'Yesh): This is like Rashi, who explains that we do not rely on women or Nochrim. We discuss Stam (i.e. not doctors).

Kaf ha'Chayim (56): Eliyahu Rabah says that this opinion is primary.

Mishnah Berurah (29): Since a Yisrael is commanded about Shabbos, and he says to be Mechalel, surely he relies on his expertise.

Kaf ha'Chayim (57): Maharil Vali says that nowadays that no one may heal without permission of the Nochri Chachamim, anyone who engages in healing is considered an expert. Birkei Yosef says that he discussed his era.

Kaf ha'Chayim (58): The Rashbatz says that if a Nochri doctor spoke l'Fi Tumo (unaware of the consequences) that Ploni is dangerously sick, it depends on what Ploni says and those (Yisre'elim) present. If they see that he is not dangerously sick, we are not Mechalel Shabbos. We do not rely on the Nochri. If there is a Safek, we are Mechalel.

Shulchan Aruch (618:1): If a Choleh needs to eat, if there is an expert doctor, even a Nochri, who says that if we do not feed him he will worsen and become dangerously sick, we feed him based on the doctor.

Magen Avraham (1): The Rosh brought from Riva that a Nochri or woman is believed to contradict a Yisrael.

Mishnah Berurah (1): Even though in OC 328, the Rema brings an opinion that Stam Yisre'elim are believed, but not Stam Nochrim, unless he is an expert, all agree that we rely on women regarding childbirth, for they are expert about it.

Shevus Yakov (3:20): Some say that even one who cannot smell may bless on spices, for even though his body does not benefit from it, his Neshamah benefits. Even though doctors say that he has no benefit from at all, since their books are based on Nochri Chachamim such as Aristotle and his colleagues, we do not rely on them for Halachah. Yefei Mareh in Berachos, and Teshuvas Chavos Ya'ir (219) say so.

Kaf ha'Chayim (1): Shevus Yakov says that we do not rely on expert doctors regarding Halachah, but when there is concern for danger, we rely on them (Yafeh l'Lev).

Bi'ur Halachah (DH Choleh): Tif'eres Yisrael (Yoma 8:26) says 'nowadays we must consider whether we rely on Nochri doctors. I have seen them abandon their reputation. They say about any mild illness that it is (mortally) dangerous to fast. The Ramatz and Ru'ach Chayim say so even about Yisrael doctors. Many are suspected to transgress Torah law and Shabbos, and they do not fast due to Apikursus. This requires great investigation whether one may rely on them. It depends on the Rav's judgment.

Kaf ha'Chayim (551:148): Ru'ach Chayim (4) says that if a Yisrael doctor transgresses Torah law or Shabbos, he is not believed to say that one must eat meat (during the week of Tish'ah b'Av) or that he cannot fast. Anyone suspected about a matter is not believed to judge it or testify about it. A Nochri is believed more than he is.

Shulchan Aruch (YD 155:1): We do not use a Nochri to heal any sickness with mortal danger, unless he is a professional, for we are concerned for murder. This is even if it is a Safek if the Choleh will die. If he will Vadai die, one may be healed by a Nochri.

Shach (1): The Mechaber connotes that if there is no danger, it is permitted in every case. Rashi, the Rif and Rambam say so. The Rosh and Tur forbid a matter without sickness or danger, such as bloodletting. Since there is no profession for this cure, if he kills him, they will not attribute it to lack of knowledge. Rather, they will say that it was his time to die, or that the doctor hated him and killed him. He will not ruin his reputation regarding others. Tosfos brings this from R. Tam, and the Ran, R. Yerucham and the Agudah conclude like this.

Gra (1): Tosfos, the Rosh and other Poskim hold that the answers in Avodah Zarah do not disagree. Rav Yehudah discusses one who is not sick. He has a mere pain. This is forbidden in every case.

Shach (3): A professional would not ruin his reputation.

Shulchan Aruch (ibid.): If he says 'medicine Ploni is good for you, and do such and such.' However, one may not take it from him. Some say that this is forbidden only if he heals for free, but if he is paid, it is permitted in every case, for he fears losing his wage.

Shach (5): Perhaps he lies in order to sell the medicine. Alternatively, we are concerned lest he mixed in poison.

Gra (7): The Gemara said that the Mishnah does not distinguish between whether or not he pays, but the law is true (it is permitted when he is paid).

Bach (YD 154:1): Even though we rely on Nochri doctors, for they would not ruin their reputations (155:1), we do not rely on a Nochri midwife, for Nefalim are common. Even if she would kill the baby, people would not say that she killed him. They would say that it was a Nefel.