This discussion concerns how to correctly place the four Parshiot that are inside the Tefillin. Rashi's opinion is that the order should look like the top left picture. Rabbeinu Tam disagrees and holds that the order as per the top right picture, with both Parshiot of Vehaia together in the middle.
These two opinions seem to be mutually exclusive and one must assume that only Rashi or only Rabbeinu Tam is right and this understanding was prevalent among the Halacha codifiers for many centuries. In practice, most rabbis sided with Rashi's opinion.
All that changed in the times of the Ari, when he explained according to Kabbalah that both views were not mutually exclusive. He said that both opinions are 100% valid and that "Eilu VeEilu Divrei Elokim Chaim / both opinions are the word of God".How can this be true?
The Shut Min Hashamain (see original here and Hebrew wiki here), a fascinating early work by a Tosafist who would literally "sleep over his questions" and get answers in his dreams, says that when he asked about this discussion is his dream he was told that both views are valid. This Teshuva became famous because of what he said next: he was also told that Hashem held like Rabbeinu Tam but that the angels sided with Rashi! A heavenly discussion! So both Rashi and R. Tam were right as there's really two ways of fulfilling the Mitzva of tefilin.
Which begs the question - if God holds like Rabbeinu Tam isn't it funny that the Halacha follows Rashi!? Shouldn't at least the Halacha follow Gods opinion??
To answer this, I point you to a famous Medrash that has become a hit song by Avraham Fried - until recently I used to blog about Jewish Music so I remember these things... That Medrash (please help me with the source but if my memory serves me well, it's a Medrash Rabba at the very end of either Shir Hashirim or Eicha) brings a dispute between God and the Jewish people about Teshuva - Hashem says that "Shuvu Eilai Veashuva", do Teshuva first and only then I will come to you. The Jewish people in turn say "Hashiveinu..Venashuva", that first God should come to us and then we will do Teshuva. The conclusion of the Medrash is that the Jewish people won the discussion because of the rule of Yachid Verabim Halacha KeRabim - if there's a discussion between one person and many others, the Halacha follows not the individulal but the many others - the "Rabim"/many.
So here too, Halacha follows the "Rabim", the angels, as they were more numerous - even though the "One" in the other side is no other the God! (that also explains why the Ari claims that the Teffilin of R. Tam are of a higher level than Rashi - it's the pair favored by God!)
But I digress so let's come back to our line of thought. The Ari says that both opinions are right.
The Ari's approach explains how Rabbeinu Tam could suddenly challenge his uncle (Rashi) and offer a different approach to the everyday Mitzva of Tefillin - this discussion preceded Rashi and Rabbeinu Tam and they merely recorded which view they sided with.
But not everyone accepted the approach of the Ari. Many rabbis continued to understand the views of Rashi and R. Tam as mutually exclusive, and the Vilna Gaon (aka Gra) was one of them. He famously stated that if you start considering other opinions you will end up doning 64 pairs of Teffilin! 64 may sound exagerated, but consider that:
- the Ra"avad proposed a third order for the Parshiot.
- there's a discussion if the Parshiot should be placed horizontally or vertically (see my post about it).
- there's a discussion on how to make the Parsha Setuma in Tefillin (see here for more on that).
- there's a discussion if the ink must be only from carbon or if it can be also from vitriol as it is the custom todat (my post on ink is here).
- there's a discussion also between Rashi and R. Tam on the shape of the letter Chet (here).
- although that's a minor issue, there's also an important discussion if you should write the Tefillin with the regular Ktav Ashurit (beit yosef) or with the Ari's Ktav (see here).
- and more...
So I guess the Gra had a solid argument there. But I did hear from a family source that the Divrei Yatziv of Zanz used to put some three other pairs of Teffilin in secrecy, so as you see there are many different approaches.
Be it as it may, the teachings of the Ari were widely adopted by many communities, and numerous Sephardic Kabbalists and specially Hasidic jews don two pairs of Tefillin everyday. With one notable difference: the Sephardic Kabbalists used to put both Tefillins at the same time (see pic below), while the Hasidim put one at the time, after each other. The Sephardim had tiny Tefillins, of less than 2 centimeters each, and that gave them enough space to put two Tefillins in the head and two in the arm.
Rabbi Ovadia Yosef (Yalkut Yosef Hil' Tefillin) in his youth gave a controversial ruling that today it's better to do like the Hasidim and not put both Tefilin simultaneously because the scribes lost the ability to make Tefillin of less than 2cm and if you put two big tefillins at the same time, one will be not be at the right place in your head and arm.
It is the prevalent Minhag that only married men should put Tefilin of R. Tam, with the notable exception of Hasidei Chabad, who put it already from the age of Bar Mitzva.
If you have to buy Tefillin of Rabbeinu Tam, I must mention that many say that you should buy from a scribe who subscribes to this opinion and puts it in a regular basis, because of a Talmudic "drasha" (see more on that here).
So which side are you on?