Writing on the Doorpost
And you shall write them (the words of the Shma) on the mezuzot (doorposts) of your houses and of your gates (Devarim 6:9).
Thus does the Torah command us in the mitzvah of mezuzah. A literal reading of this passage could lead us to assume that we are obligated to actually etch these words on the doorposts themselves. How then did the Torah communicate that the intention is to write the two Shma chapters with ink on parchment and attach it to the doorpost?
Two sources are mentioned for this. One is based on the rule of gezeirah shavah an equation between two disparate chapters which contain a similar term. The term writing used in regard to this mitzvah also appears in the chapter (Devarim 24:3) about writing a get divorce document. Just as that document can be written only on parchment so too must the mezuzah be written on parchment, not on the doorpost itself.
Rabbi Ashi, however, adds a point based on the word uchesavtam which can be read as two words meaning write and perfect. Since it is impossible to do perfect writing on a doorpost, he concludes, the intention of the Torah was for the writing to be done on parchment.
These two sources, concludes the gemara, are interdependent. If not for Rabbi Ashis point we would assume that the Torah literally insisted on writing on the doorposts themselves. But this alone would have ruled out only writing on the doorposts because it is impossible to do perfect writing in that position. There would still remain the possibility of doing such perfect writing on a detached stone and affixing it to the doorpost. The comparison to the writing of the get eliminates this option and leaves us with the mezuzah on parchment.
A Problem of Precedence
You shall bind them as a sign on your arm and they shall be as totafot between your eyes (Devarim 6:8).
In its commandment to wear tefilin the Torah issued an explicit order to first put on the tefilin shel yad the tefilin of the arm and afterwards the tefilin shel rosh the tefilin of the head. What happens if someone mistakenly first takes out of his tefilin bag the shel rosh?
This question arises because of the rule that we do not skip over mitzvos and if we have come in contact with an object for performing a mitzvah we are obligated to give it precedence to another awaiting our performance. The most common example of this is the situation of a Jew on a weekday morning putting on a talit and tefilin. The rule is that he must put on the talit first so that he will be moving up to a higher level of sanctity when he puts on the tefilin (Shulchan Aruch Orech Chaim 25:1).
But should he accidentally take the tefilin in his hand before taking out his talit he must give the tefilin precedence in order not to be guilty of skipping over mitzvos.
This rule applies, however, only when the order of the mitzvoth is not explicitly dictated by the Torah. Since the Torah insisted that tefilin must first be placed on the arm the consideration of not skipping over mitzvoth must be put aside if he took the shel rosh in his hand and it must be put aside until the shel yad is worn.
Such a conflict should, however, be avoided. This is why the gemara elsewhere (Mesechta Yoma 33b) cautions that when one takes off his tefilin he should place them in his bag in such a manner which will guarantee that he first encounters the shel yad and performs the mitzvot in the proper order. There are two opinions among halachic authorities as to how this is achieved. One view is to have a long, narrow bag in which the shel rosh is placed in back and the shel yad in front to assure that it will be first encountered. Another opinion is that it is preferable to place the two side by side (See Magen Avraham in Orech Chaim 28 and Tarei Zahav in Orech Chaim 25).
Common practice is to place them side by side with the shel rosh on the right and the shel yad on the left. It is advisable, however, to try to place them in such a manner that the shel yad protrudes slightly more so that it will be the first to be encountered.