V’asisa choshen mishpat, ma’aseh choshev, k’ma’aseh ephod ta’asenu - And you shall make a breastplate of judgement, a woven design, similar to the ephod it should be made (Shemos 28:15)
Kli Yakar points out that only two of the eight articles of clothing worn by the kohen gadol were woven from multiple fabrics: the ephod - the vest - and the choshen - the breastplate. He explains that these two articles of clothing have similar symbolic purpose that is best expressed by a mixture of fabrics woven together. The ephod was worn as a means of achieving kapparah for sins of avodah zarah while the choshen was worn as a means of achieving kapparah for sins of perverting justice. Neither of these shortcomings need be overt; indeed they can be violated by thought alone. Thus, if one mentally accepts the validity of avodah zarah, one is liable even though one has not actually served an idol. Similarly, if a judge did not adjudicate a case correctly, or if he did not think things through to the extent necessary, he will have sinned even though his misdeed has no physical manifestations. The Hebrew ma’aseh choshev - translated in this context as woven - can also mean the product of thought; thus the connection between the symbolism [kapparah] and essence [woven] would seem to be mainly alliterative.
The choshen mishpat - the breastplate worn by the kohen gadol - was, as we have already noted, a source of atonement for the sins of judges who adjudicated incorrectly. However, it had an additional role as well. The choshen, and more specifically the stones that were inset within it, served as a means of divining the truth in a given situation. When the urim ve-tumim - a parchment on which the name of G-d was inscribed - was placed within the folds of the choshen, a question could be raised and the stones would provide the answer by illuminating the appropriate letters inscribed upon each one. According to the Midrash, the stones had a total of seventy two letters [the names of each individual tribe as well as the words shivtei yisrael so that each letter of the Hebrew alphabet was represented].The ability to decipher the answer demanded special talent, for the letters did not appear one at a time allowing for an easy reading [see Ramban]. According to the Vilna Gaon, Eli ha-kohen misread the answer when he queried the choshen about Chana [mother of Shmuel who Eli saw praying at the Mishkan at Shilo]. The letters heh, kaf, resh and shin all lit up and Eli read them as shikorah [drunkard] when he should have read k’sheirah [fitting].
Interestingly, the parameters as to when the choshen was to be used to are not delineated. It would seem from the Talmud in Yoma [73b] that the choshen was only used for public questions [e.g., questions similar to that posed by Yehoshua as to which tribe should lead the armies into Canaan at the time of the first conquest]. Obviously, the kohen gadol could not use it to ask for the numbers of the weekly lottery pick or as a means of playing the stock market. This limitation, however, would not account for the question addressed by Eli regarding the sobriety of Chana which would seem to have no public manifestations. It is also not clear whether consultation with the choshen was voluntary or mandatory. One might think that if there was a means of determining the Divine will, then the practice of relying upon the choshen should have been a requirement! Moreover, if all that was needed was to address the choshen, why not raise every single query to it and thus insure the possibility of mistake in halachah?
Ramban writes that the choshen was a lower form of prophecy, just above the bat kol used during the period of the Second Beit ha-Mikdash. As such, it could not be used at will, just as any other prophecy [with the exception of Moshe] was dependent upon situations and conditions. Moreover, in halachic matters just as a bat kol has no standing [see for example Bava Metzia 59b] we can assume that the choshen would also not have any standing. This might well explain why the choshen was considered the permanent kappara for a beit din that had erred in a halachic decision and the connection might well be more than a play on the words ma’aseh choshev..