We learn about the mitzvah of tefillin in Parshas Devarim, Perek Vov Posuk ches. The gemara in Brachos, daf chof gimmel, amud bais says in the name of Rav Yitzchok that if a person goes to eat a seudas keva, (full fledged meal) he should first remove his tefillin and then proceed to eat.
There are two reasons why a person should not eat a seudas keva while wearing his tefillin. Firstly, Rashi and the Meiri explain the reasoning for this is due to the fact that a person may get drunk during his meal. The Tiferes Yisroel in Meseches Yodayim, Perek Bais, mishna daled, seif koton lamed heh is medayek from the words of Rashi that if one eats and drinks things that are not meshaker, he may eat while wearing tefillin. The second tirutz is due to kalus rosh; just like a person is not allowed to eat in a Bais Hakneses, he is prohibited from wearing tefillin while eating.
The Rishonim argue about the topic of eating while wearing tefillin. The Meiri holds that as long as one wears tefillin, he may not eat even a seudas arei (temporary meal.) The Rambam in Hilchos Tefillin, Perek Daled, halacha tes zayin says that one is permitted to eat a seudas arei while wearing tefillin. Halacha lemaaseh, we pasken that achilas kevawhile wearing tefillin is assur and achilas arei while wearing tefillin is muttar, based on the Shulchan Aruch, Orach Chaim, siman mem, seif koton ches.
The shiur of achilas arei is a kebeitza, which is the same as the shiur found in the discussion of not eating in a succah, as mentioned in Hilchos Succah.
The Mishna Berura in siman mem, seif koton yud tes brings down a safek in the name of the Shulchan Shlomo. The fact that one may eat a seudas arei while wearing tefillinapplies to those who wear tefillin the entire day; however, those who only wear tefillin during davening are prohibited from wearing tefillin while eating a seudas arei.
The Shailos U’Tshuvos, Teshuras Shai, chelek alef, siman resh tes vov discusses this further. Since in our times we don’t have the minhag of wearing tefillin the entire day, the Shulchan Aruch paskens that one may eat a seudas arei in tefillin. The Chasam Sofer used to drink coffee after davening while wearing his tefillin. We could explain that since the Chasam Sofer would wear tefillin after davening while giving his shiur, it was considered as if he wore them the entire day, and he therefore was permitted to eat a seudasarei while wearing tefillin.
The question arises concerning a gemara in Kiddushin, daf samech vov, amud alef. The gemara tells us that Yanai Hamelech would wear the tzitz during his meal. Tosfos, in Yuma, daf ches, amud alef says that the tzitz was holier than tefillin since it had Hashem’s name on the outside. How was Yanai Hamelech able to eat a meal while wearing tefillin?
According to Rashi and the Meiri, we could answer that those meals had no wine or anything to render him drunk, so he was able to eat his meal while still wearing the tzitz. According to the second tirutz mentioned above, that it could bring to kalus rosh, we could say that he did not eat bread at all, but rather he ate vegetables, so it was not a seudas keva.
We see the importance of having the right frame of mind when wearing tefillin. May we all be zocheh to wear tefillin with the proper kavonos.