OPINIONS: The Gemara states that one may not remove his mind from his Tefilin while he wears them. This prohibition is derived from the Tzitz of the Kohen Gadol. The Torah commands that the Tzitz must be "constantly (Tamid) on his forehead" (Shemos 28:37), which means that the Kohen Gadol may not remove his mind from the Tzitz while he wears it. The Gemara derives a Kal v'Chomer from the Tzitz to the Tefilin: if one must keep his mind on the Tzitz, which has only one name of Hash-m on it, then certainly one must keep his mind on the Tefilin, which has many names of Hash-m written in it. (According to Tosfos (8a, DH u'Ma), this Kal v'Chomer is only an Asmachta mid'Rabanan.)
What is considered a "Hesech ha'Da'as," a mental interruption, from one's Tefilin while he wears them?
(a) The SHA'AGAS ARYEH (#39) records two opinions. The first opinion is that of RABEINU YONAH, as cited by the ROSH in Berachos (3:28). Rabeinu Yonah points out that the Gemara in Sukah (26b) explicitly states that one is permitted to take a short nap ("Shinas Arai") while he wears Tefilin. Why is a short nap permitted? It should be considered a "Hesech ha'Da'as," as one cannot concentrate on his Tefilin while he is asleep.
Rabeinu Yonah answers that it must be that "Hesech ha'Da'as" does not mean that one must keep his mind on the Tefilin constantly. Rather, it means that one may not act lightheadedly and frivolously while he wears the Tefilin. As long as one's mood is somber and he conducts himself with awe of his Creator, his lack of conscious focus on his Tefilin it is not considered a "Hesech ha'Da'as." One who dozes off with his Tefilin upon him is not acting frivolously. On the contrary, while he dozes he is less aware of the frivolities of this world, and thus dozing is not considered a "Hesech ha'Da'as."
Rabeinu Yonah adduces support from the words of the RAMBAM that a "Hesech ha'Da'as" while one wears Tefilin is defined as a mood of lightheadedness ("Kalus Rosh"). The Rambam (Hilchos Tefilin 4:25) writes that while a person wears Tefilin, he stands in awe of Hash-m and is not drawn after gaiety and idle chatter.
(b) The second opinion cited by the Sha'agas Aryeh is that of the RAMBAN in TORAS HA'ADAM (and as cited by the TUR YD 388). The Ramban writes that a mourner may not wear Tefilin because his grief prevents him from focusing on the Tefilin and thus causes a "Hesech ha'Da'as." Similarly, the RAMBAM (Hilchos Tefilin 4:13) writes that a person who is in distress and whose mind is not settled is exempt from the Mitzvah of Tefilin because he will not be able to concentrate properly on them.
The Sha'agas Aryeh points out that the Ramban and Rambam apparently argue with Rabeinu Yonah's understanding of a "Hesech ha'Da'as," because a person in mourning or in distress certainly is not in a frivolous mood. They define "Hesech ha'Da'as" as a lack of focus on the Tefilin, unlike the way Rabeinu Yonah defines "Hesech ha'Da'as." Others point out that this also appears to be the opinion of TOSFOS in Shabbos (49a, DH she'Lo Yishan), who says that a person may not sleep in Tefilin because sleeping ("Shinas Keva") is a "Hesech ha'Da'as."
The Sha'agas Aryeh notes that these Rishonim do not mean that one must keep his mind on his Tefilin at all times, because that is impossible. Rather, they mean that one must not take his mind off of the Tefilin for more than a certain amount of time. The Sha'agas Aryeh asserts, based on the Gemara in Sukah (26a) cited by Rabeinu Yonah, that the maximum amount of time that one may remove his mind from his Tefilin is the time of a short nap ("Shinas Arai"), which is defined as the time in which person can walk one hundred Amos. Before that amount of time passes, one must return his attention to the Tefilin. Based on the assumption that the average person walks one Mil (2000 Amos) in eighteen minutes, the time in which a person walks one hundred Amos is 54 seconds. (According to the Chazon Ish's calculation that one walks four Amos in two to three seconds, the time of "Shinas Arai" is 50 to 75 seconds.)
The Sha'agas Aryeh proves that "Hesech ha'Da'as" is unrelated to frivolity from the fact that the prohibition of "Hesech ha'Da'as" from one's Tefilin is derived from the Tzitz. If "Hesech ha'Da'as" refers to a frivolous mood, then the verse which prohibits the Kohen Gadol from taking his mind off of the Tzitz is unnecessary, because the Tzitz is worn only in the Beis ha'Mikdash where lightheadedness is forbidden altogether (Berachos 54a). Moreover, even outside of the Beis ha'Mikdash lightheadedness is proscribed, as the verse states, "Lest you forget Hash-m, your G-d" (Devarim 8:11; see also Avos 3:13).
(c) A third opinion not cited by the Sha'agas Aryeh appears to be that of the TOSFOS YESHANIM (8a) who says that the prohibition of "Hesech ha'Da'as" with Tefilin refers to passing flatulence. He understands "Hesech ha'Da'as" to mean an act that is disrespectful to the Tefilin.
HALACHAH: As mentioned above, the TUR (YD 388) cites the Ramban's opinion, that severe distress is considered a "Hesech ha'Da'as." However, the SHA'AREI TESHUVAH (OC 28:1) points out that it is nearly impossible to avoid removing one's concentration from his Tefilin for the amount of time the Sha'agas Aryeh mentions, and he quotes the Gemara that says that "the Torah was not given to angels" (Berachos 25b, Yoma 30a). He concludes, therefore, that one may rely on the opinion of Rabeinu Yonah, who says that "Hesech ha'Da'as" is defined as frivolousness. In fact, the Tur himself (in OC 44) cites the opinion of Rabeinu Yonah when he rules that one must be careful not to be frivolous while he wears Tefilin.
How, though, are the questions that the Sha'agas Aryeh asks on the opinion of Rabeinu Yonah to be resolved, and how can the apparently conflicting rulings of the Tur (who cites both the Ramban and Rabeinu Yonah) be reconciled?
Perhaps Rabeinu Yonah's intention is as follows. When the Torah commands that the Tzitz must be "constantly (Tamid) on his forehead" (Shemos 28:37), it does not command the Kohen Gadol to avoid lightheadedness, because the requirement to avoid lightheadedness is obvious and needs no additional verse. Rather, the Torah commands that not only must the Kohen Gadol avoid all lightheadedness, he also must take precautionary measures to prevent himself from coming to lightheadedness. Focusing constantly on the Tzitz is one such precautionary measure. Similarly, when one wears Tefilin he must take precautionary measures to prevent himself from coming to lightheadedness. This requirement to take precautionary measures applies only to the Kohen Gadol while he wears the Tzitz and to a man while he wears Tefilin. It does not apply to someone who merely stands in the Beis ha'Mikdash (he must avoid lightheadedness, but he does not have to take precautionary steps to that effect).
With regard to the apparently contradictory rulings of the Tur, perhaps Rabeinu Yonah does not disagree with the Ramban. It is possible that he does not limit the prohibition of "Hesech ha'Da'as" specifically to "lightheadedness." Rather, perhaps he means that one may not let his mind become totally involved in worldly matters while he wears the Tefilin (or Tzitz). Rabeinu Yonah may agree that one may not wear Tefilin while he is in a state of mourning or overcome with grief. He mentions the state of "lightheadedness" only as an example of a mood of one who is overcome by his emotions and does not focus on matters of Kedushah. (Rabeinu Yonah mentions lightheadedness only as an example of what a person might need to avoid by focusing on the Tefilin constantly. A person does not need to attempt to avoid feelings of mourning or grief, because there is no reason for an emotionally healthy person to be overcome suddenly with such feelings. Moreover, focusing on the Tefilin will not necessarily help to avoid those feelings. When the Ramban and Rambam mention mourning and grief as forms of "Hesech ha'Da'as," they are discussing the act of donning Tefilin in the first place when one is already overcome with mourning or grief.)
This is also the opinion of the RITVA in Sukah (26a) who says (in his answer for why one may sleep "Shinas Arai" with Tefilin) that while one wears Tefilin he may not "turn his attention to mundane matters," but he is not required to consciously focus on his Tefilin at all times. This might also be the intention of the TOSFOS YESHANIM here (8a, DH Tefilin) in his second explanation.
Even TOSFOS in Shabbos (loc. cit.), who clearly argues with Rabeinu Yonah and prohibits one from sleeping with Tefilin because of "Hesech ha'Da'as," may agree that one is not required to think about the Tefilin at every moment. When one goes to sleep with Tefilin he actively renders himself unable to think about his Tefilin, and thus that act is considered a "Hesech ha'Da'as." (A short nap, though, is not considered a "Hesech ha'Da'as," because one does not render himself unable to think about his Tefilin for a significant duration of time.) As long as one is awake and in a solemn mood with the fear of G-d, his mind is not considered to be removed from his Tefilin even though he might not actually be thinking about them at every moment.
In conclusion, the mental state indicated by "Hesech ha'Da'as" is defined in three different ways: 1. being overcome with emotion or frivolity, which distracts one's focus from fear of Hash-m (Rabeinu Yonah); 2. actively putting oneself in a state in which it is not possible for him to think about Tefilin (Tosfos in Shabbos); 3. acting in a disrespectful manner towards the Tefilin (Tosfos Yeshanim).
It is worth noting that the MAGEN AVRAHAM (OC 44:2) cites the ruling of the BACH who says that it is a "Mitzvah Min ha'Muvchar" not to remove one's concentration from the Tefilin at all. (M. KORNFELD)