Introduction:

In Shabbos (49a), Rav Yanai only permits a person whose "body can be kept as clean as that of Elisha Ba'al Kenafayim" to wear Tefilin. Abaye says this means he can control himself from inadvertently passing gas while wearing the Tefilin; Rava explains this means he can control himself from inadvertently falling asleep with the Tefilin.

On what point are Abaye and Rava arguing? They cannot be arguing over whether one must control himself from passing gas when wearing Tefilin. Rava certainly would agree that if a person cannot keep himself from passing gas while wearing Tefilin he should not wear them, for it would be a disgrace to the Tefilin! Besides, since it is easier to control the passing of gas than to keep oneself from falling asleep, it is implicit in the words of Rava that one must not only control himself from passing gas, but also from sleeping inadvertently.

It would therefore seem that their argument is over sleeping with Tefilin. Abaye does not require that a person control himself from sleeping with them, while Rava does.

Question:

But Abaye himself prohibits sleeping in Tefilin in our Gemara, so why should he argue with Rava in Shabbos?

Answer:

Abaye's argument with Rava (in Shabbos) is not whether we permit a person to sleep with Tefilin -- it certainly is not permitted. Rather, he argues that even if one cannot prevent himself from inadvertently falling asleep, he may still wear Tefilin. The reason for this is that sleeping is not inherently forbidden; it is only forbidden due to the concern that one might pass gas while sleeping. According to Abaye, we are not concernted that he might fall asleep, and when asleep he might pass gas.

Chag Same'ach,

Mordecai Kornfeld