Shiur and Sources available: here.
This week I dealt with the question of how Halacha deals with manuscripts and archeological evidence. More specifically, how does Halacha respond when it turns out that manuscripts indicate that the texts of Gemara and Rishonim that Halacha has been based on are flawed, new commentaries of Rishonim are printed from manuscript that were not (heavily) used in Halacha, or archeological evidence appears to support a Halachic position.
Conceptually, these are related. The two extreme positions would be that 1) Halacha developed the way it did for a reason, and new material is no reason to change it. 2) Halacha automatically needs to incorporate the new material in some way. Both of these positions seem unlikely to be true.
The first position, which Daniel Sperber unfairly ascribes to the majority of the rabbinic establishment, is somewhat of a straw man. Even poskim who are reticent to accept this type of evidence cite positions from the Meiri, Gaonim, etc. whose manuscripts were printed relatively recently. If not that, they are willing to cite teshuvot of the Rambam that were not available to earlier poskim. There are exceptions, but not as many as he would have us think.
It is unarguable that Rishonim used manuscripts to figure out the correct text of the Gemara. It is also clear that the Rama thought that new positions that were discovered could in theory be used to overturn previous psakim, arguing that poskim might have reconsidered their positions if they new about the earlier authorities who argued. What people debate is whether psak now should be as accepting, or reject this evidence.
The closest to the rejectionist position is that of the Chazon Ish, or at least that ascribed to him. His position has been dealt with in three articles in tradition, the most recent being : THE ROLE OF MANUSCRIPTS IN HALAKHIC DECISION-MAKING: HAZON ISH, HIS PRECURSORS AND CONTEMPORARIES by Moshe Bleich: Tradition, Vol. 27, No. 2 (Winter 1993), pp.22-55. Two of the articles, this one by Bleich and Zvi A. Yehuda’s in Tradition 18:2, ascribe a somewhat extreme position to the Chazon Ish, claiming that he felt Halacha had been guided by Hashgacha, and therefore the psakim in Shulchan Aruch remained true even new manuscripts appeared.[R. Moshe Shternbuch extends this argument to include poskim later than Shulchan Aruch as well.] Shneur Leiman, in a rejoinder to Yehuda’s article, argued that the Chazon Ish’s position was not as rejectionist as it has been made out to be.
Technical arguments have also been raised, namely that manuscripts can also have mistakes, so a new text does not necessarily prove anything. Even for those who are more accepting, this does give one pause before blindly accepting new positions.
Many poskim have been much more accepting. For example:
For many centuries, people relied on the position of Rashi that an area was not considered a reshut harabim if it did not have 600,000 people, assuming that this was the majority position. Mishkenot Yaakov noted that in his day, enough manuscripts of Rishonim were available to know this was the minority position. Thus, he rejected it. Beit Efraim responded, based on the Tumim. The Tumim noted that Rov does not apply to historical majority, because there is no way of knowing what the Rov really is. Many poskim have lived whose writings have been lost or who never wrote at all. Thus, we need to rely on other rules of psak. Beit Efraim argues, therefore, that the existence of new Rishonim makes no difference. God guided psak so poskim accepted Rashi’s position, and that position remains decisive regardless of new evidence. Note that even Mishkenot Yaakov is willing to be machmir, but is not clear about being mekal.
Shulchan Aruch HaRav argues that we should use many kinyanim to sell Chametz, as new Rishonim indicate the number of disputes that arose as to what kinyanim worked. R. Moshe Shternbuch argues that we cannot even accept a Chumra against Shulchan Aruch and rejects Shulchan Aruch HaRav.
Thus, we see Shulchan Aruch HaRav and Mishkenot Yaakov are willing to accept new Rishonim, while Bet Efraim and R. Shternbuch are more reticent.
R. Ovadiah is very clear that new Rishonim and new manuscript evidence can be used to overturn earlier positions, even that of Shulchan Aruch (who he usually takes as binding). Note again that no one thinks new manuscripts or Rishonim always can be used to overturn old psak, just that in theory they are part of the equation.
A related issue is archeology. There, poskim can raise the same fundamental and technical arguments as above. Here, I used the example of Tefillin and Techeilet.
Smag writes that support can be brought for using Rashi Tefillin from pairs that were discovered in the Kever Yechezkel. Bach and Drisha, however, argue that it may have been buried because it was pasul (this is unlikely, as one could simply switch the order of the parshiyot). At any rate, one sees the two sides of the issue.
The Gemara tells a story of Rabbah Bar Channah who saw the people who died in the Midbar (the generation of Moshe). He tried and failed to take their tzitzit in order to determine how one should tie the tzitzit, an issue debated by Beit Hillel and Beit Shammai. When he returns, he is rebuked for not simply looking and recording the details.
R. Schachter proves from here that Halacha can recognize archeological evidence. For this reason, he accepts the modern tcheilet. Famously, the Rav did not, claiming that in this case mesorah was necessary. R. Aviner and R. Chaim Kanivesky argue more fundamentally. The claim that the fact that Rabbah Bar Channah did not think to record the details shows that God was opposed to the use of archeology in Halacha.
For more on these issues, see the Tradition articles, as well as R. Jachter’s articles (part one is here, the rest follow), in Grey Matter 3, and the article in Techumin cited by R. Jachter.