Shadal's theory about the Urim veTumim

Summary: As a sort of alphabet oracle, similar to Chazal but slightly different.

Post: In parshat Tzav, we hear of the Urim veTumim, placed into the choshen:

8. And he placed the choshen upon him, and he inserted into the choshen the Urim and the Tummim.

ח. וַיָּשֶׂם עָלָיו אֶת הַחֹשֶׁן וַיִּתֵּן אֶל הַחֹשֶׁן אֶת הָאוּרִים וְאֶת הַתֻּמִּים:

According to Rashi:

		את האורים: כתב של שם המפורש:

It would seem that this written-down Ineffable Divine Name somehow empowered the communication via the letters on the stones with the names of the shevatim on the choshen.

Thus, from Yoma 73b:

 למה נקרא שמן אורים ותומים אורים שמאירין את דבריהן תומים שמשלימין את דבריהן 

And further:

כיצד נעשית רבי יוחנן אומר בולטות ריש לקיש אומר מצטרפות והא לא כתיב בהו צד"י אמר רב שמואל בר יצחק אברהם יצחק ויעקב כתיב שם והא לא כתיב טי"ת אמר רב אחא בר יעקב שבטי ישורון כתיב שם מיתיבי כל כהן שאינו מדבר ברוח הקודש ושכינה שורה עליו אין שואלין בו שהרי שאל צדוק ועלתה לו אביתר ולא עלתה לו שנאמר (שמואל ב טו, כד) ויעל אביתר עד תום כל העם וגו' סיועי הוה מסייע בהדייהו:

Or, the latter in English:

(f) Question: How would the answer appear?
(g) Answer: R. Yochanan says the letters would stand out (in their place) and Resh Lakish says they would organize into the words.
(h) Question: But the names of the Shevatim are missing the letter Tzadi?
(i) Answer: The names Avraham Yitzhok and Yakov also featured on the Choshen.
(j) Question: But we are still missing a Tes?
(k) Answer: The words Shivtei Yeshurun appeared there, as well.
(l) Question: We see that the operation of the Urim VeTumim was based on the level of Shechinah in the Kohen Gadol and so (according to Rashi) there is no need for the letters to stand out or to organize at all!?
(m) Answer: The Kohen would assist the Urim VeTumim to function.

I've also seen the idea of the letters lighting up, rather than protruding. (Thus, Urim.) See also this.

Shadal has a similar, though different theory:

ח ויתן אל החשן את האורים ואת התומים: משה נתנם בחושן בשעת לבישה; אם כן, לא היו קבועים בחושן; אם כן, היה הכהן משתמש בהם לפרקים, ומוציאם מתוך החושן; ואולי היו כ"ב אותיות חקוקות על כ"ב חתיכות קטנות של עץ או של מתכת, והיה הכהן מרים משם אות אחת ואחר כך אות אחרת, והשגחת ה' היתה מזמנת לו תשובה נכונה לשאלתו; והיו האותיות ההן נקראות כל אחת בשם מיוחד, האל"ף היתה נקרת "אוּר", והת"ו היתה נקראת "תֹם", וכללן, מאל"ף ועד ת"ו, נקראו "אורים ותומים", ומזה נמשכת קבלת חז"ל (יומא עג:) שמאותיות שמות השבטים היו מבינים התשובה. 

"and he inserted into the choshen the Urim and the Tummim" -- Moshe inserted them into the choshen at the time of wearing it; it so, they were not affixed in the choshen; if so, the kohen made use of them at odd intervals, and {at that point} removed them from within the choshen. And perhaps they were 22 letters engraved on 22 small strips of wood or metal, and the kohen lefter from there one letter, and after that another letter, etc., and Hashem's Providence arranged for him a correct answer to his question. And these letters were each called by a specific name, the aleph was called `ur {which begins with an aleph}, and the tav was called tam {which begins with a tav, and perhaps signifies completion}, and the group of them, from aleph until tav, were called urim veTumim, and from this derived the tradition of Chazal {in Yoma 73b} that from the letters of the names of the shevatim they understood the answer."

This is a common rationalist-Jewish approach to explain miracles. Rather than have every miracle usurp the natural order, make it operate within the existing order, but with the guidance of Divine Providence. Shadal also has going for him that the roshei teivot are aleph and tav. And in fact, alphabet oracles are known to exist in the ancient world. This could have been influenced by the Jewish practice -- they took it from us! -- or it could be co-opting an existing oracular practice, with the change that this one actually worked.

I wonder if the modern invention of computers and different output displays would have influenced Shadal to treat the standard, traditional explanation as more plausible, rather than a corrupted understanding of the way the alphabet oracle worked. Probably not, since part of the idea is one of non-overt Divine guidance.