The 'ovot and yed'onim
(יט) וְכִֽי־יֹאמְר֣וּ אֲלֵיכֶ֗ם דִּרְשׁ֤וּ אֶל־הָאֹבוֹת֙ וְאֶל־הַיִּדְּעֹנִ֔ים הַֽמְצַפְצְפִ֖ים וְהַמַּהְגִּ֑ים הֲלוֹא־עַם֙ אֶל־אֱלֹהָ֣יו יִדְרֹ֔שׁ בְּעַ֥ד הַחַיִּ֖ים אֶל־הַמֵּתִֽים׃ (כ) לְתוֹרָ֖ה וְלִתְעוּדָ֑ה אִם־לֹ֤א יֹֽאמְרוּ֙ כַּדָּבָ֣ר הַזֶּ֔ה אֲשֶׁ֥ר אֵֽין־ל֖וֹ שָֽׁחַר׃
(19) Now, should people say to you, “Inquire of the ghosts and familiar spirits that chirp and moan; for a people may inquire of its divine beings—of the dead on behalf of the living— (20) for instruction and message,” surely, for one who speaks thus there shall be no dawn.
(לא) אַל־תִּפְנ֤וּ אֶל־הָאֹבֹת֙ וְאֶל־הַיִּדְּעֹנִ֔ים אַל־תְּבַקְשׁ֖וּ לְטָמְאָ֣ה בָהֶ֑ם אֲנִ֖י יְהוָ֥ה אֱלֹהֵיכֶֽם׃
(31) Do not turn to ghosts and do not inquire of familiar spirits, to be defiled by them: I YHWH am your god.
(ג) וּשְׁמוּאֵ֣ל מֵ֔ת וַיִּסְפְּדוּ־לוֹ֙ כָּל־יִשְׂרָאֵ֔ל וַיִּקְבְּרֻ֥הוּ בָרָמָ֖ה וּבְעִיר֑וֹ וְשָׁא֗וּל הֵסִ֛יר הָאֹב֥וֹת וְאֶת־הַיִּדְּעֹנִ֖ים מֵהָאָֽרֶץ׃
(3) Now Samuel had died and all Israel made lament for him; and he was buried in his own town of Ramah. And Saul had forbidden [recourse to] ghosts and familiar spirits in the land.
וטעם להזכיר האובות וידעונים. בעבור המת וכן כתוב בישעיה בעד החיים אל המתים. האבת. מגזרת וכאובות חדשים כי הם עיקר זאת האומנות:
Having brought up the dead, Scripture now mentions charmers and wizards , since they purport to communicate with the dead (as it is written in Isaiah, “[seek guidance] of the dead on behalf of the living” [Isaiah 8:19]). charmers [Hebrew: ’ovot] as in “jugs” [Hebrew: ’ovot] [Job 32:19] — this being a major tool of their trade.
Robert Alter on אבות וידענים (from 1 Samuel 28:3)
The two Hebrew terms are generally paired, and both refer to the spirits of the dead. The latter is derived from the verbal root y-d-' "to know." The ghosts and familiar spirits are linked metonymically with the necromancers who call them up, but the terms themselves primarily designate the spirits. Biblical views about postmortem existence tend to fluctuate. Often, the dead are thought to be swallowed up in "the Pit" (she'ol) where they are simply silenced, extinguished forever. Sometimes, the dead are imagined as continuing a kind of shadowy afterlife in the underworld, rather like the spirits of the dead in Book 11 of the Odyssey. Following on this latter view, necromancy in the ancient Hebrew world is conceived not as mere hocus-pocus but as a potentially efficacious technology of the realm of spirits, which however, has been prohibited by God, Who wants no human experts interfering in this realm.
Richard Elliot Friedman in his Commentary to Leviticus 19:31:
There is arechaeological and textual evidene that ancient Israelites engaged in ancestor veneration, i.e., worship at the site of the tombs of known relations. The Israelite priests were opposed to this, and the biblical law codes forbid it. It appears to have come to an end during the reign of Hezekiah, king of Judah. My colleague Baruch Halpern has shown that it was an ironic consequence of the Assyrians' destruction of the Israelite countryside, which cut the people off from these traditional sites.
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It has been suggested that the etymology of the term 'ob can be sought in the Ugaritic phrase il 'ib, usually understood as cognate to Hebrew elohe abiw but plausibly meaning "god of the pit" rather than "god of the father(s).
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Although the biblical references to life after death are often noted to be fewer than one would have expected, the fact remains; these references plus the archaeological evidence show that Israelites in biblical times believed in an afterlife.
From Ancestor Worship: Is it Biblical? by Choon Sup Bae and PJ van der Merwe, http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0259-94222008000300009&lng=en&tlng=en
Watts (1985:126) says that this Scripture also contains a fairly derogatory reference to the practices of necromancy when it describes the diviners/mediums/spirits who "chirp and mutter" . This implies a garbled gibberish which the necromancer utters in his/her trance-like state. The text explicitly refers to people who consult the dead and therefore to the belief that the dead have the ability to help the living. This was necessary since the Ancient Near East (including Israel) was drawn to divination as much as any other group of nations in the history of mankind. The context here suggests that Isaiah had to defend his prophetic calling and role against diviners and spiritualists.
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Gehman (1999:152) refers to Gesenius who defined a medium as someone with "a familiar spirit". The Hebrew word ob denotes in its simplest terms, "a leather bottle" which was typically used for water or wine. It later also denoted a "necromancer, sorcerer, conjurer who professes to call up the dead by means of incantations [magic words] and magic formulas, in order that they may give response as to doubtful or future things.".
This clearly compares the medium to a leather bottle, filled with a spirit. From the belly of the medium come the gurgling, bubbling sounds of the spirit which possessed him/her.
Here's something interesting in light of the above;
The Septuagint's translation for אוב from Leviticus 19:31:
εγγαστριμύθοις - "gastromancy". gastro=stomach.
Gastro/belly in Latin is venter. "Speech" in Latin is "loqui." Anything sound familiar?
Here is an excerpt from Wikipedia:
Originally, ventriloquism was a religious practice. The name comes from the Latin for to speak from the stomach, i.e. venter (belly) and loqui (speak). The Greeks called this gastromancy (Greek: εγγαστριμυθία). The noises produced by the stomach were thought to be the voices of the unliving, who took up residence in the stomach of the ventriloquist. The ventriloquist would then interpret the sounds, as they were thought to be able to speak to the dead, as well as foretell the future.