Theology as Seen in Prophets and Psalms/Source Two

The sacred lot by means of which the ancient Hebrews were wont to seek manifestations of the Divine will. Two other channels of Divine communication were recognized, viz. dreams and prophetical utterance, as we learn from numerous passages of the Old Testament. The three forms are mentioned together in 1 Samuel 28:6. "And he (Saul) consulted the Lord, and he answered him not, neither by dreams, nor by priests (Hebrew, Urim, LXX delois), not by the prophets." There can be no doubt that in this instance the Douay translation of "priests" is wrong, based on the mistaken rendering "sacerdotes" of the Latin Vulgate. The etymological signification of the words, at least as indicated by the Masoretic punctuation, is sufficiently plain. Urim is derived from the Hebrew for "light", or "to give light", and Thummim from "completeness", "perfection", or "innocence". In view of these derivations it is surmised by some scholars that the sacred lot may have had a twofold purpose in trial ordeals, viz. Urim served to bring to light the guilt of the accused person, and Thummim to establish his innocence. Be that as it may, the relatively few mentions of Urim and Thummim in the Old Testament leave the precise nature and use of the lot a matter more or less plausible conjecture, nor is much light derived from the ancient versions in which the term is subject to uncertain and divergent renderings.

http://www.newadvent.org/cathen/15224a.htm