Gemara

Question: Why do we go out to the cemetery on a Ta'anis?

Answer #1 (Levi bar Chama or R. Chanina): We show that we are considered like Mesim in front of Hash-m.

Answer #2 (the other of Levi bar Chama and R. Chanina): We ask the dead to request mercy for us.

Berachos 18b (Beraisa): A case occurred, a Chasid gave Tzedakah on Erev Rosh Hashanah in a famine year; his wife was angry. He spent the night in a cemetery, and heard the spirits of two deceased girls conversing. He heard that hail will destroy anything planted in the first Revi'ah (winter rainfall). He planted in the second Revi'ah. Everyone else's crops were destroyed; his crops were not. He lodged in the cemetery the next Erev Rosh Hashanah, and again heard about punishments and saved himself from them.

The Chasid went again the next year. The spirits did not discuss the punishments because they realized that the living had heard their words.

A case occurred, Ze'iri deposited money with an innkeeper; the innkeeper died. Ze'iri went to Chatzermaves (a cemetery), and asked the innkeeper where the money was. The innkeeper told him.

A case occurred, orphans' money was deposited with Shmuel's father. Shmuel was not there when his father died; people were accusing him of taking orphans' money. He went to Chatzermaves, and asked to speak with his father. He asked where the orphans' money is.

His father: It is in the millstone. The upper and lower coins are ours, those in the middle belong to the orphans.

Mo'ed Katan 28a: Rava's brother asked Rava to appear to him after death. He did. He asked him what the pain of death was like, and Rava answered. Rava had a similar dialogue with Rav Nachman before and after Rav Nachman died.

Sanhedrin 65b (Beraisa): Doresh El ha'Mesim is one who starves himself and spends the night in a cemetery so that Ru'ach ha'Tum'ah will come upon him.

Rishonim

Rambam (Hilchos Avodah Zarah 11:13): Doresh El ha'Mesim is one who starves himself and spends the night in a cemetery in order that a Mes will come to him in a dream and answer his questions. Others use special clothing, incantations and incense and sleep alone in order that a particular Mes will speak to him in a dream. The general rule is, anyone who does an act in order that a Mes will inform him is lashed - "Lo Yimatzei Becha... v'Doresh El ha'Mesim".

Hagahos Maimoniyos (8): The Re'em (89, or 334-335 in Yere'im ha'Shalem) says that if Reuven was sick and Shimon made him swear to return to Shimon after death to answer questions, this is not Doresh El ha'Mesim, for he only seeks the Ru'ach. The Ru'ach is not called Mes, only the body is. The Isur is to use Kishuf, e.g. Ov and Yid'oni, to conjure up the dead from the grave. This permitted a Chasid, Shmuel, Rava and his brother to seek to overhear or speak with the dead.

Rosh (Sanhedrin 7:5): Doresh El ha'Mesim is one who starves himself and spends the night in a cemetery so that Ru'ach ha'Tum'ah will come upon him.

Poskim

Shulchan Aruch (YD 179:13): Doresh El ha'Mesim is one who starves himself and spends the night in a cemetery in order that Ru'ach ha'Tum'ah will come upon him.

Shulchan Aruch (14): One may ask a Choleh to swear to return after death to answer questions.

Rema: Some permit imposing an oath even after death, as long as he does so to the Ru'ach, not to the body.

Rebuttal (Beis Yosef DH u'Mah she'Chosav v'Doresh): The Re'em learns from the Chasid and Shmuel that this is permitted. We cannot learn from the Chasid, for he went to the cemetery to evade arguing with his wife without having to tell others. He did not intend to seek the dead. Even though the Beraisa does not say why he went again the next year, perhaps it was again to evade his wife.

Note: It seems that also the third time he went on Leil Rosh Hashanah (one spirit suggested going to hear the decrees). Every episode with a Chasid refers to R. Yehudah ben Bava or R. Yehudah b'Rebbi Ila'i (Bava Kama 103b). It is difficult to say that one of them needed to evade his wife three times on Rosh Hashanah!

Beis Yosef: Even if he went to hear decrees, he did not intend that they tell him anything; he just wanted to hear what they would say. We can say that Shmuel contacted his father while he was awake, through Shemos (of Hash-m). We find that Rav and Shmuel visited a cemetery and 'did what they did' to determine what caused the Mesim to die. Doresh El ha'Mesim is forbidden only if one does an act, e.g. starves himself or wears special clothing or burns special incense and sleeps alone. If he does not do an act, but speaks with the Mes through Shemos while he is awake, perhaps this is not Doresh El ha'Mesim. We do not distinguish seeking the body or the Ru'ach. This distinction of the Re'em is dubious. A body by itself is nothing (it is powerless)! Nevertheless, we learn from Mo'ed Katan that one may ask a Choleh to swear to return after death to answer questions. Even though Rava did not impose an oath, if with an oath it is Doresh El ha'Mesim, the same would apply without an oath.

Defense (Levush): The Re'em's distinction is not dubious. All Chachmei ha'Kabalah know, and it says often in the Zohar, that when a person dies and his Neshamah and Ru'ach leave, immediately all powers and forms of Tum'ah seek to enter him and cling to him. They desire to resemble the pure Neshamah that left him. This is why the Tum'ah of a dead Yisrael is greater than that of a dead animal or Nochri. Therefore, if one seeks the body of a dead Yisrael, he clings with those powers of Tum'ah and is Metamei himself. The same applies to the Ru'ach of dead Nochrim, for they are called dead even while alive. However, even seeking the Ru'ach of dead Yisraelim is permitted only through oaths, but not through actions. One who wears special clothes... for this purpose is lashed for Doresh El ha'Mesim.

Prishah (23): Doresh El ha'Mesim is only when one desires Ru'ach ha'Tum'ah. The Chasid desired only to hear what the Mesim say.

Beis Yosef (DH Kosav Rabeinu): R. Yerucham says that asking a Mes even though oaths is Doresh El ha'Mesim. It seems that he merely teaches that one is liable without starving himself or other actions, but he agrees that one may put such an oath on a Choleh. Surely, Rava did not transgress!

Darchei Moshe (4): The Beis Yosef rejected the Re'em's proof from Shmuel by saying that Shmuel used (Shemos that impose) oaths. R. Yerucham forbids this! Rather, we must say like the Re'em, that one may seek the Ru'ach.

R. Chayim Palti'el (brought in Bach YD 217 b'Sof DH Mi): A vow to go to the cemetery is like Doresh El ha'Mesim. Kalev went to Chevron to prostrate on the graves of Tzadikim to help Tefilah be received. We find that it is proper to go to the grave of someone whom one disgraced and prostrate on it, to show honor. But one who does not have one of these intents has no reason to go. I help him to annul his vow.

Rebuttal (Bach): The custom is for people to go, and no one protests. The Zohar supports this. He should pray that Hash-m help him in the merit of his ancestors.

Kitzur Shulchan Aruch (128:13): The custom is to go to the cemetery on Erev Rosh Hashanah to prostrate on graves of Tzadikim, give Tzedakah and supplicate to arouse the Tzadikim ha'Kedoshim in the ground to advocate on our behalf on Yom ha'Din. Also, the burial site of Tzadikim is Kadosh and Tahor and Tefilah is received well there. One asks Hash-m to have mercy on him in the merit of the Tzadikim; he does not put his hope in the Mesim, for this is close to Doresh El ha'Mesim.

Ha'Kosev (on Berachos 18b, in Ein Yakov 49b): We cannot understand these episodes simply, that Tzadikim sought to communicate with the dead, for this is forbidden! Rather, it was all through dreams.

Igros Moshe (OC 5:43): Poskim argue about whether or not one may request from angels that they fulfill their missions in the best way for us. All forbid to request from them anything else. A Mes cannot pray, but it is possible for his Neshamah to pray. Seemingly, the Neshamah is like an angel without any mission, so all the more so we may not request from it. Mesim are exempt from Mitzvos, so we cannot ask them to pray for us. However, their children or others can ask Hash-m to answer prayer in the merit of Mesim, for the dead are pained by the pain of their children or others they knew, and certainly by the pain of Kelal Yisrael. This is a reason to be more lenient about asking Mesim. We arouse them to pray for us to alleviate their own pain. Also, perhaps they are more sensitive to human pain than angels are. The Or Zaru'a holds like the first side; he holds that Doresh El ha'Mesim includes requesting from the dead. The Bach permits, like the second side. He holds that Doresh El ha'Mesim is only doing actions to bring Ru'ach ha'Tum'ah on himself. The Beis Yosef brings only what is surely permitted (imposing an oath on a Choleh) and surely forbidden (actions to seek the Mesim). He left contradictory Diyukim about oaths on Mesim (Sa'if 13 permits, for it is not an action; Sa'if 14 forbids, for it is after death) to show that he was unsure, so Chachmei ha'Torah may decide the issue.