Responsa על ראש השנה 33:8

I also asked [in my dream] regarding the commandment of tefillin. We have learned¹see Mechilta d'Rabbi Yishmael 13:9:7: "One who wears tefillin is like one who reads from the Torah, and one who reads from the Torah is exempt from wearing tefillin." We are unsure about the meaning of this phrase. Does it mean that one is truly exempt from wearing tefillin if one reads from the Torah, or studies even a single halacha or a single verse? For most people regularly read a halacha or a biblical verse. Or does it mean that at the actual moment that one is reading from the Torah, one is then exempt from putting on tefillin, because of the principle that someone engaged in one commandment is exempt from another²cf. Sukkah 26a:2 . Tosafot on Sukkah 10b:5 makes the precision that only if the fulfilment of one commandment would lead to the annulment of another is this principle applied? Moreover, I was unsure regarding those who claim that wearing tefillin requires a cleanliness of the body comparable to Elisha (Shabbat 49a:6), and therefore exempt themselves on this ground. Yet perhaps all that is required is not to flatulate or to sleep while wearing them³This is the opinion in Teshuvot Maharam 649 and of Rabbeinu Tam cited in Tosafot on Rosh Hashanah 17a:10. See also Siman 26 above.. We were also unsure regarding what was taught (Rosh Hashanah 17a:10): "Sinners of Israel who sin with their bodies - these are those skulls that do not don tefillin," which was interpreted by the commentators to refer to those who never wear tefillin at all. I asked about this [in my dream], whether it was as they claimed, that if one wore tefillin even once a week they would not be considered a sinner; or whether it is as is written in the literature, that the term applies to anyone who could be wearing them and isn't⁴See Rabbeinu Tam, ibid.. I asked all these questions together.

Q. Is a person who has had a nocturnal pollution permitted to put on the phylacteries?
A. The custom is widely accepted to permit such a person to study the Torah, to recite his prayers, and to put on his phylacteries.
SOURCES: Cr. 37; cf. L. 223–4.