John Calvin on Exodus 28:30

30. And thou shalt put in the breastplate. From these words some infer that the Urim and Thummim were distinct from the whole work, which is before described; others think that they were the twelve stones, because no mention will be made of them when Moses relates that the whole was completed. But nothing is more probable, as I have already said, than that on the breastplate itself some representation was given of light in doctrine, and of entire uprightness of life; and therefore after Moses has called it “the breastplate of judgment,” he also speaks of it as “the judgment of the children of Israel;” by which expression he means a certain and defined system, or an absolutely perfect rule, to which the children of Israel ought to direct and conform themselves.