The proof (circumstantial, not direct, but still, to my mind, convincing) is in a very strange position that Rambam takes at the very beginning of Hil. Sanhedrin. He holds that the mitzvah of Shoftim Ve-shotrim is only in the Land of Israel. That's problematic because it goes directly against a Gemara in Makkot 7a. Attempts to suggest that the Rambam had a different version of the text fall short, as many of the commentaries point out.
However, the following progression would make perfect sense:
- The Gemara in Sanhedrin states that Semikha can only be performed in the Land of Israel, though once administered, it grants powers everywhere.
- Semicha, for the Rambam (again, this is clear from his discussion of semicha in both the Yad and the Sefet Ha-Mitzvot; see also Sefer Ha-Chinukh on Shoftim Ve-shotrim) is the mechanism by which the mitzvah of minui Shoftim is perpetuated.
- Therefore, the Rambam can understand the inability to administer semicha outside the lant of Israel not as a procedural impossibility, but as the result of the fact that outside of Israel, there's simply no mitzvah to engage in that mitzvah, rendering that act meaningless. The source for Rambam's strange psak would seem to be the sugyot that deal with semicha outside of Israel.
- The limitation ofthis mitzvah to the Land of Israel has an implication that we're not just interested in finding a way to adjudicate claims, but to set up a hierarchy of courts. In other words, for the Rambam, the mitzvah is not to set up courts, but to create a court system. This mitzvah devolves specifically upon Jews living in Israel.
- Thus, absence of the original method of appointing judges does not absolve the population from the obligation to set up courts.
- At this point, one may ask why the Rambam insisted that there must be another way to reinstitute the Sanhedrin. The answer might be from the predictions in Isaiah, the promise of the coming of a Sanhedrin-appointed Messiah, or simply that it was inconcievable to the Rambam that a mitzvah as essential as providing justice might be completely neglected on what basically amounts to a technicality. The Rambam therefore 'de-mystifies' semicha by understanding it as an act of appointment and not the conferral of some kind of special status.