The Urim and Thummim (Heb. אוּרִים וְתֻמִּים) was a priestly device for obtaining oracles. On the high priest's
(an apron-like garment) lay a breastpiece (חֹשֶׁן) – a pouch inlaid with 12 precious stones engraved with the names of the 12 tribes of Israel – that held the Urim and Thummim (Ex. 28:15–30; Lev. 8:8). By means of the Urim, the priest inquired of YHWH on behalf of the ruler (Num. 27:21; cf. Yoma 7:5, "only for the king, the high court, or someone serving a need of the community"); they were one of the three legitimate means of obtaining oracles in early Israel (Urim, dreams, prophets; I Sam. 28:6). Owing to the oracular character of the Urim, the breastpiece is called "the breastpiece of decision" (חֹשֶׁן הַמִּשְׁפָּט). (The concept evokes "the Tablets of Destiny" in Babylonian mythology – the symbol of supreme authority that lay on the breast of the chief god; Pritchard, Texts, 63, 67, 111.) The right to work this oracle was reserved for the levitical priests (Deut. 33:8).
Occasionally the term ephod is used with reference to the Urim-oracle associated with it (I Sam. 14:3, 18 [according to LXX]; 23:6, 9; 30:7). The latest period for which there is evidence of use of the ephod-Urim is that of David (but cf. Hos. 3:4); subsequently, oracles are conveyed exclusively by prophets. In postexilic times, when the Urim oracle was extinct, difficult questions were reserved "until a priest would appear with Urim and Thummim" (Ezra 2:63; Neh. 7:65; cf. Sot. 9:12: "After the former (i.e., pre-exilic) prophets died, the Urim and Thummim became extinct"; and Josephus (Ant. 3:218), who avers that the oracle ceased 200 years before his time).
There is no biblical information on the appearance of the Urim, the material out of which they were made (the Samaritan text of Ex. 28:30; 39:21 adds a command to manufacture the Urim and tells of its execution), or the technique of their use. The most illuminating passage is the Greek of I Samuel 14:41, whose underlying Hebrew is mutilated in the received texts: הָבָה תָמִים, conventionally rendered "Give a perfect answer":
Saul said: "O YHWH God of Israel, why have you not answered your servant this day? If the guilt be in me or in my son Jonathan. O YHWH God of Israel, give Urim (הָבָה אוּרִים). But if this guilt is in your people Israel, give Thummim (הָבָה תֻמִּים)."
(For a defense of the received Hebrew, however, see M. Tsevat. in Sefer Segal (1955), 78–84.)
From the use of the verbs hippil and nilkad in connection with the Urim (verses 41–42), it appears that they were a kind of lot ((marked) stones or sticks?), since these verbs occur in connection with the casting of lots (Isa. 34:17; I Sam. 10:20). They were suitable for indicating which of two alternatives was right; hence inquiries to be decided by them were designed to elicit "yes" or "no" answers (I Sam. 23:10–12;30:8).
The etymology of the terms is obscure. From the Greek passage adduced above, it seems that the two terms are the names of two objects. Hence the conjecture that Urim derives from ʾ arar, "curse," and Thummim from tammam, "be whole," indicating negation and affirmation respectively. Tradition has connected the first term with light (ʾ or) or instruction (Aram. ʾ oraita). Thus both Greek δήλωσις καὶ ʾ αλήθεια, "declaration/revelation and truth," and Vulgate doctrina et veritas, "teaching and truth," treat the pair as a hendiadys meaning true instruction – with reference to the oracle. (An apparent derivative with this meaning is the novel coinage אורתום, found in the Qumran Thanksgiving Psalms Scroll with reference to divine illumination (4:6, 23; 18:29; see J. Licht's commentary to 4:6)). A talmudic interpretation finds an allusion to the unequivocalness of the oracle in two words: "They cause their message to be lucid [מאירין]… and carried out [משלימין]" (Yoma 73b).
The earliest speculation on the technique of the oracle is reflected in Josephus (Ant. 3:217), who states that victory was forecast by the shining of the stones in the breastpiece. The talmudists fancied that the oracle was spelled out by the miraculous protruding of letters out of the tribal names engraved on the stones (Yoma 73b; Maim. Yad, Kele ha-Mikdash, 10:11). Rashi takes account of the separate existence of the Urim by making them a document bearing the tetragrammaton, whose presence inside the breastpiece insured the clarity and perfection of the oracle (comment to Ex. 28:30 and Yoma 73a; cf. Samuel b. Meir, who calls the Urim a kind of conjuration by
divine names). Naḥmanides (at Ex. 28:30) combines the various strands of interpretation: the Urim was a text bearing divine names placed inside the breastpiece, by virtue of which various letters out of the tribes' names lit up; the Thummim were other divine names by whose virtue the priest was able to combine the letters perfectly into the divine message.