Kedushat Tefillin

The kedushah of the Tefillin is very great, for so long as the Tefillin are on a person’s head and hand he is humble, he has yiras Shamaim, and he is not drawn after frivolity, empty chatter or evil thoughts. He turns his heart to truth and justice. (Rambam Hil. Tefillin. 4:25)

While wearing Tefillin, a person is forbidden to turn his thoughts away from the Tefillin for even a moment. The kedushah of Tefillin is greater than the kedushah of the Tzitz (worn on the forehead of the Cohen Gadol), for the Tzitz was inscribed with one name of G-d while the name of G-d appears 21 times in the passages contained in the Tefillin. We learn from the verse, “And it will be on his forehead always,” (Shemos 28:38), that the Cohen is forbidden to turn his thoughts away from the Tzitz. Since the kedushah of the Tefillin is even greater than the kedushah of the Tzitz, we can infer that this prohibition also applies to Tefillin—both the Tefillin of Rashi and Rabbeinu Tam. Since the name of G-d appears many times in the Tefillin shel yad, it is forbidden to turn one’s thoughts away from them after removing the Tefillin shel rosh.
There are those who hold that a person is considered to have turned his thoughts away from the Tefillin when he forgets that he is wearing them for the amount of time it takes to walk 100 amos. But the poskim cite the Rambam who holds that it is forbidden to forget about them for even a moment.
What does it mean to forget the Tefillin? There are two basic opinions among the Rishonim. Some hold that as soon as a person is unaware that he is wearing Tefillin, he has forgotten them. Others hold—and this is the halachah--that we only say a person has forgotten the Tefillin if he falls into laughter and frivolity, or becomes so preoccupied with mundane matters that he is no longer conscious of yiras Shamayim. A person who entertains bad thoughts, smokes, or releases intestinal gas is also said to have forgotten that he is wearing Tefillin. Nevertheless, the mitzvah of wearing Tefillin is best fulfilled when a person is constantly aware of them, except while learning Torah and saying the Shmoneh Esrai.
Chassidim and anshei maase are careful not to speak at all while wearing Tefillin to avoid the possibility of being drawn into chattering and frivolity.
There is disagreement among the poskim whether the prohibition of turning the mind away from the Tefillin is from the Torah or rabbinical. There are poskim that hold that a person who forgets that he is wearing Tefillin does not fulfil the mitzvah.
According to the Beis Yosef, the prohibition of forgetting the Tefillin also applies to a person who holds them in his hand, but most poskim disagree. Nevertheless, it is forbidden for a person to sleep while holding Tefillin because he may drop them while sleeping.
Strictly speaking, it is permitted drink and to eat a casual meal while wearing Tefillin, but the poskim write that this applies only to a person who wears Tefillin all day. A person who wears Tefillin only while davening may not drink or eat at all while wearing them.
A baal nefesh will be careful not to touch anything dirty while wearing Tefillin and not to kill lice, etc.
The Arizal writes (Sefer Hakavanos) that there is no greater p’gam (blemish) than to forget Tefillin while wearing them. The Chidah (Avodas Hakodesh) writes that a person should be very careful about remembering the Tefillin while wearing them, and that even though it is very difficult, the punishment for forgetting them is very great. The Baahash on The Kitzer Shulchan Orach writes that forgetfulness of the Tefillin is the ultimate source of all our troubles in these times, for the reward for fulfilling the mitzvah of wearing Tefillin is that “all the nations of the world will see the name of G-d upon you and be afraid of you.” When we forget the kedushah of the Tefillin and turn our thoughts away from yiras Shamayim, the gentiles become hostile.