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An excerpt here:
Rav Yehudai Gaon (c. 750 CE)
1) One outspoken promoter of the popularisation of Tefilin was Rav Yehudai Gaon. He claimed that the old notion of a “clean body” only applied to those who wanted to defy the bans against wearing Tefilin during the Roman persecutions and thus risk their lives. In other words, only exceptionally holy people were permitted to endanger their lives by defying the bans. However, such measures were no longer applicable in the more peaceful Gaonic times. Therefore everyone, regardless of their state of physical and spiritual cleanliness was now obligated to wear Tefilin since the Roman persecutions had long passed.
2) Additionally, Rav Yehudai Gaon argues, strict spiritual cleanliness was also no longer demanded during Gaonic times, because we still read from the Torah despite not being perfectly or ritually clean. If we can read from the Torah in technically ‘impure’ states, we can similarly wear Tefilin.
Thus, the old limiting conditions requiring a “clean body;” 1) if one wanted to defy the bans during Roman persecution, and 2) if one claimed that general “impurity” still exempted one from Tefilin during Gaonic times –no longer apply. Now every male from the age of thirteen must don Tefilin regardless of the circumstances.
Rav Yosef Gaon (c. 850 CE)
It seems that Rav Yehudai Gaon’s noble attempt to reinstate the neglected mitzva of Tefilin was not entirely successful. A century later, in the time of Rav Yosef Gaon (the father of Rav Saadia Gaon), it was still thought that Teflin was only for exceptionally holy people and not for the common folk.
The following question was addressed to Rav Yosef Gaon:
“A merchant involved in business, should he put on tefillin during prayer (Shemona Esrei) and Shema, or perhaps only a great person puts them on, while one who is not such an important person doesn’t, so as not to appear haughty, since the entire congregation does not put them on” (cited in Kanarfogel (1976:110), emphasis is mine).
Rav Yosef Gaon had to again emphasise that the obligation to wear Tefilin fell on all people, common and holy alike.