Gemara

Question: Why was R. Meir allowed to learn from Acher?

(Rabah bar bar Chanah): "...v'Sorah Yevakshu mi'Pihu Ki Mal'ach Hash-m Tzevakos Hu" - seek Torah from someone only if he is like an angel!

Answer #1 (Reish Lakish): R. Meir expounded "Hat Oznecha u'Shma Divrei Chachamim v'Libcha Tashis l'Dati" (even if Chachamim are not like Hash-m wants, hear their words, but follow Hash-m's will).

Answer #2 (R. Chanina): He expounded "Shim'i Vas u'Re'i v'Hati Oznech v'Shichchi Amech" (hear their words but ignore their deeds).

Question: These verses contradict the first verse!

Answer: A small person must learn from an angel. A great person may learn from anyone.

Answer #3 (Rav Dimi): R. Meir ate the fruit and discarded the shell (he absorbed from Acher only what is proper).

(Rava): "El Ginas Egoz Yoradti" - Chachamim are compared to nuts because even if a nit is sullied with mud and excrement, the inside is not ruined. Likewise, even if a Chachamim sinned, his Torah is not despised.

Eliyahu told Rabah bar Shila that Hash-m was reciting teachings of all the Chachamim except for R. Meir, because he learned from Acher.

Rabah: But R. Meir ate the fruit and discarded the shell!

Eliyahu: (Hash-m accepted your words, and) now He is saying 'Meir my son says, when a person is in pain the Shechinah says 'My head hurts, my arms hurt'. The Shechinah feels pain over the blood of Resha'im, all the more so over the blood of Tzadikim!'

Moed Katan 17a: There was a Chacham of ill repute. Rav Yehudah was reluctant to excommunicate him, for Rabanan needed to learn from him. But it was Chilul Hash-m not to excommunicate him! He asked Rabah bar bar Chanah.

(Rabah): "...v'Sorah Yevakshu mi'Pihu Ki Mal'ach Hash-m Tzevakos Hu" - seek Torah from someone only if he is like an angel!

Rav Yehudah excommunicated him.

Rishonim

The Rif and Rosh (Moed Katan 9a and 3:11) brings the episode in which Rav Yehudah excommunicated the Chacham.

Rambam (Hilchos Talmud Torah 4:1): If the Rebbi does not go in a good path, even if he is a great Chacham and everyone needs him, we do not learn from him until he repents. "...V'Sorah Yevakshu mi'Pihu Ki Mal'ach" - seek Torah from someone only if he is like an angel.

Rav Sheinberg, Shlita: An angel has only one mission (see Rashi Bereishis 18:2). One should seek Torah from someone only if he has only one mission: to help the Talmid learn (wages and recognition do not concern him).

Poskim

Shulchan Aruch (YD 246:8): If the Rebbi does not go in a good path, even if he is a great Chacham and everyone needs him, we do not learn from him until he repents.

Question: The Gemara (Chagigah 15b) said that a Gadol can learn from a improper Chacham. Why didn't the Rambam and Shulchan Aruch bring this?

Answer #1 (Shach 8): Tosfos says that Rav Yehudah excommunicated the Chacham because Ketanim learned from him, and perhaps they would be influenced. If in the days of the Gemara there were Ketanim, all the more so nowadays we are all considered Ketanim!

Answer #2 (Lechem Mishneh, and Shach 8, citing his father): Rabah bar bar Chanah's teaching is brought often in Shas, without qualification. Also, it says that R. Meir expounded a verse to permit learning from Acher. This connotes that we disagree, and hold that all are forbidden. This is why Hash-m was not reciting R. Meir's teachings. Answer #1 is not valid for the Rambam, for he brings Halachos even for Yemos ha'Moshi'ach, when people will be Gedolim!

Rebuttal #1 (Aruch ha'Shulchan 22): If the Gemara disagrees with R. Meir, it should have said 'this is wrong'!

Rebuttal #2 (Pri Chodosh (in Likutim in Frankel Rambam): The final answer, Rav Dimi, said that R. Meir ate the fruit and discarded the shell. It does not distinguish between big and small Talmidim. Hash-m agreed with this, and recited R. Meir's teachings afterwards.

Note: The Pri Chodosh does not explain why the Rambam and Shulchan Aruch omit the Heter (for anyone) to eat the fruit and discard the shell.

Rivash (45 DH Emnam): The Rambam cites "Hat Aznecha..." at the beginning of Moreh Nevuchim. He himself relied on this Heter to learn Aristotle's philosophy after he had learned all of Torah. His purpose was to be able to resolve difficulties for Yisraelim who had learned philosophy and were confused by it. Unfortunately, it caused him to err and explain a few matters to agree with philosophy. Perhaps this was to placate the confused, but some things he wrote should not be heard. Also R. Levi was great in Torah, and learning philosophy caused him to err greatly in many matters. We should take a lesson from these two giants and avoid learning things counter to Torah. We may not assume that we will be able to take the fruit and discard the peel.

Birkei Yosef (9): The Rambam holds that the Heter is only for one who is awesome in Chachmah. He knew that this applies to himself, but he did not write the Heter lest smaller people consider themselves qualified. Alternatively, he holds that even Chachamim permit a Gadol ha'Dor if it is known that he has learned the entire Torah. He omitted the Heter because it is not explicit in the Gemara, or lest people err.

Kovetz (in Likutim in Frankel Rambam): The Rambam holds that the Halachah is like the Sugya in Moed Katan, unlike R. Meir. However, one may rely on R. Meir in pressed circumstances, e.g. to refute Apikorsim.

Ha'Kosev (in Ein Yakov): In Moed Katan it seems that (Rav Yehudah wanted to excommunicate the Rebbi because) even Gedolim may not learn from an improper Rebbi.

Or ha'Chayim (Devarim 12:28): R. Meir expounded correctly, that a Gadol may learn from a Rasha. However, people did not know the Heter. Our deeds must be free of suspicion from people.