(light and perfection). When the Jewish exiles were met on their return from Babylon by a question which they had no data for answering, they agreed to postpone the settlement of the difficulty till there should rise up "a priest with Urim and Thummim." (Ezra 2:63; Nehemiah 7:65) The inquiry what those Urim and Thummim themselves were seems likely to wait as long for a final and satisfying answer. On every side we meet with confessions of ignorance. Urim means "light," and Thummim "perfection." Scriptural statements. --The mysterious words meet us for the first time, as if they needed no explanation, in the description of the high Priest’s apparel. Over the ephod there is to be a "breastplate of judgment" of gold, scarlet, purple and fine linen, folded square and doubled, a "span" in length and width. In it are to be set four rows of precious stones, each stone with the name of a tribe of Israel engraved on it, that Aaron "may bear them on his heart." Then comes a further order. In side the breastplate, as the tables of the covenant were placed inside the ark, (Exodus 25:16; 28:30) are to be placed "the Urim and the Thummim," the light and the perfection; and they too are to be on Aaron’s heart when he goes in before the Lord. (Exodus 28:15-30) Not a word describes them. They are mentioned as things-already familiar both to Moses and the people, connected naturally with the functions of the high priest as mediating between Jehovah and his people. The command is fulfilled. (Leviticus 8:8) They pass from Aaron to Eleazar with the sacred ephod and other pontificalia . (Numbers 20:28) When Joshua is solemnly appointed to succeed the great hero-law-giver he is bidden to stand before Eleazar, the priest, "who shall ask counsel for him after the judgment of Urim," and this counsel is to determine the movements of the host of Israel. (Numbers 27:21) In the blessings of Moses they appear as the crowning glory of the tribe of Levi: "thy Thummim and thy Urim are with thy Holy One." (33:8,9) In what way the Urim and Thummim were consulted is quite uncertain. Josephus and the rabbins supposed that the stones gave out the oracular answer by preternatural illumination; but it seems to be far simpler and more in agreement with the different accounts of inquiries made by Urim and Thummim, (1 Samuel 14:3,18,19; 23:2,4,9,11,12; 28:6; Judges 20:28; 2 Samuel 5:23) etc., to suppose that the answer was given simply by the word of the Lord to the high priest comp. (John 11:51) when, clothed with the ephod and the breastplate, he had inquired of the Lord. Such a view agrees with the true notion of the breastplate.
Urim and Thummim
What were the Urim and the Thummim? Scripture names them but never describes them. They were to be kept in the “breastplate of judgment” on Aaron’s heart when he went in before the LORD, a way for Israel to seek an answer. After Joshua died, the people asked the LORD who should go up first against the Canaanites. Later, Saul asks and hears nothing, neither by dreams, nor by Urim, nor by prophets. Returning exiles in Ezra wait for a priest to use them. Many questions about how they worked stay open, and the text leaves them there.
5 verses
Exodus
And thou shalt put in the breastplate of judgment the Urim and the Thummim; and they shall be upon Aaron's heart, when he goeth in before the LORD: and Aaron shall bear the judgment of the children of Israel upon his heart before the LORD continually.
Deuteronomy
And of Levi he said, Let thy Thummim and thy Urim be with thy holy one, whom thou didst prove at Massah, and with whom thou didst strive at the waters of Meribah;
Judges
Now after the death of Joshua it came to pass, that the children of Israel asked the LORD, saying, Who shall go up for us against the Canaanites first, to fight against them?
1 Samuel
And when Saul enquired of the LORD, the LORD answered him not, neither by dreams, nor by Urim, nor by prophets.
Ezra
And the Tirshatha said unto them, that they should not eat of the most holy things, till there stood up a priest with Urim and with Thummim.
From Smith's Bible Dictionary
William Smith, 1884