When I lived in Israel, I often got around by hitchhiking (what the Israelis call "tremping"). Once, I got a ride with a man who looked at me and said "Hey, aren't you the tefillin expert? What do you think of the guy with the new kind of tefillin?" I expressed surprise at this, as i was unaware that there was a new kind of tefillin. He said "You know, that guy. I think his name is Rabbenu Tam". I tried hard not to laugh. This "guy" was Rabbi Yaakov Tam (1100-1171); the greatest of all Ashkenazi Talmudists and Halachists, and some argue the greatest...period. An older contemporary of RAMBAM, they share the spotlight as the seminal figures in Judaism of that time. But they were very different. While RAMBAM would pour over ancient texts, deciding which was or wasn't authoritative, and analyzing earlier commentaries in an attempt at determining which tradition was "right", and which needed to be jettisoned, Rabbenu Tam would little consider earlier views, and decided on the basis of a careful analysis of the Talmudic text or texts, attempting to come up with a reconciliation between views in the Talmud itself.
In the area of Tefillin, there was a seemingly minor debate, which threatened to divide the Jewish people. The Talmud, as we have seen, puts great emphasis on the mitzvah of Tefillin. It stresses that the four passages of the Torah contained in the Tefillin, must be written and placed in the proper order. But the Talmud is remarkably unclear as to what that order is! In rabbinic literature not included in the Babylonian Talmud, two interpretations, and practices, emerged. One was that the four passages must be in the order in which they occur in the Torah. This is borne out by an early Midrash called "Mechilta", and appears to have become the accepted practice in Babylon (Iraq). The other view is that the last two passages need to be in reverse order. Although the relevant section of the Jerusalem Talmud has been lost, several early post-Talmudic rabbis report that this was, in fact, codified in the Jerusalem Talmud, and was, in fact, the custom of the Land of Israel. For centuries, communities...and rabbis, were divided on the "right" practice. The great 11th century rabbi and commentator, RASHI, championed the view that the Torah order should be observed, and the alternative  opinion could be ignored. He was joined in this view half a century later by RAMBAM, who reports that the tefillin he had from his youth were, in fact, the alternative order, but, when he had delved into the sources, "realized" that this was not valid, and made new tefillin for himself. Ironically, RASHI's own grandson, Rabbenu Tam, differed. He accepted the alternative view as valid, and became that view's champion. Ever since, these are known as "RASHI's tefillin, and Rabbenu Tam's tefillin, respectively. Entire regions of the Jewish world were choosing between these views, which began to take on the appearance of two forms of Judaism, similar to what we would expect had there been two traditions of which day was Shabbat. By the time the Shulchan Aruch (Code of Jewish Law) was written in the late 16th century, the view of RASHI had prevailed...but had never really been determined to be the correct view. RASHIs view had become the universal custom, but doubts lingered. The Shulchan Aruch rules that all must wear RASHI's tefillin. However, one who is scrupulous in fulfilling the laws of the Torah, should wear both. If possible, they should be worn together (I have friends who do this), or else to pray in RASHI's, but to put on Rabenu Tam's later (this is my practice). However, the Shulcah Aruch warns that this should not be done by everyone, as it appears ostentatious. Only those known in the community for their piety should do this, with everyone else relying on the customary acceptance of the view of RASHI. (I won't get into the minority views of one 18th century sage that only RASHI's are valid, and one twentieth century sage that only Rabbenu Tam's are valid, but that we must also wear RASHI's as a custom. The vast majority of rabbis accept the ruling of the Shulchan Aruch).
The Kabbalists, however, saw the entire issue in a different light. They are BOTH valid (and necessary), and represent two different perceptions of G-d. How is this interpreted, and what are the implications for practice? That will be my next post.