And in the breastpiece of judgment you shall put the Urim and the Thummim, and they shall be on Aaron's heart, when he goes in before the LORD. Thus Aaron shall bear the judgment of the people of Israel on his heart before the LORD regularly.
Exodus 28:30
The Urim and the Thummim are fascinatingly mysterious phenomena, and we wish that more was known about them. What is known is that they were means of ascertaining the mind of the Lord in matters of guidance. The Hebrew words may be rendered 'Lights and Perfections', and there may be an association of ideas in Psalm 43, in the words, 'O send out thy light and thy truth; let them lead me'. There are a number of references in the Old Testament to the use of these 'instruments' which make it clear that their use was for ascertaining the mind of the Lord (cf Numbers 27:21; Deuteronomy 33:8; 1 Samuel 23:9-12, 28:6, 30:7, 8; Ezra 2:63; Nehemiah 7:65). The distribution of these references shows that there seems to have been no occurrence of Urim and Thummim guidance between the time of the early monarchy and the post-exilic period. In the prophetic age there would have been no need of them, since God was always raising up men of His choice to say, 'Thus saith the Lord'. It has been held that the Urim and the Thummim were two stones set in the breastplate of judgment, and that when the divine guidance was sought one or other of the stones glowed with a supernatural radiance signifying the Lord's will. It may be that something of this nature is indicated in 1 Samuel 14:41, which in the RSV reads, 'If this guilt is in me or in Jonathan my son, O Lord, God of Israel, give Urim; but if this guilt is in thy people Israel, give Thummim'. On the other hand, others have held that this speaks of the casting of lots (see AV rendering of the verse). However, if the casting of lots was involved in the Urim and Thummim, it could not have been done in the ordinary sense of the term. By casting lots, you could always get a reply one way or the other; but in 1 Samuel 28:6 we are told 'The Lord answered him not, neither by dreams, nor by Urim, nor by prophets'. But see Proverbs 16:33. All interpretation here must necessarily have an element of conjecture in it, since we do not in fact know what the Urim and Thummim were, nor how they worked; and it may be the wisdom of the Holy Spirit in having concealed this from us to indicate that we are not meant to have guidance in this way now, but have a more sure word of prophecy (2 Peter 1:19).