In this week's Torah-portions we learn that the Jews constructed the Mishkan and prepared the bigdei kehunah. Although in both cases the Jews followed the directions they had been given, the Torah stresses at every stage in the preparation of the bigdei kehunah that the work was done "ka'asher tzivah Hashem et Moshe" -- "just as Hashem commanded Moshe" (see Ramban 37:8, 38:22). Why would the Torah stress the fact that the Jews obeyed their orders only regarding the bigdei kehunah? Why not include the phrase by the construction of the Mishkan as well?
To answer this question R. Velvel Soloveichick explained that the verb "tzivah" is a halachic term; whenever it appears in the Torah it denotes the presence of a mitzvah le-dorot (an obligation that is binding throughout the generations), as opposed to a hora'at sha'ah (an obligation that was binding only for a limited time). This distinction between perpetual mitzvot and temporary obligations was noted by tana'im in the Sifra and was quoted by Rashi (Vayikra 6:1); the Rambam also adopted this as a criterion for inclusion in his Sefer ha-Mitzvot (Shoresh 3). For this reason the Torah only used the phrase "ka'asher tzivah Hashem et Moshe" regarding the bigdei kehunah, whose specifications would never change. It would be inappropriate for the Torah to use the term "tzivah" when referring to the details of the Mishkan, whose boards and curtains would later be superseded by the stones of the Beit ha-Mikdash.
The distinction between mitzvot le-dorot and hora'ot sha'ah should not be limited to the realm of biblical exegesis. It is relevant today as well, particularly in the area of "ma'aseh rav" (attempting to determine current policy based on the practices and recorded opinions of our Rabbeim). Developing a p'sak requires careful analysis of a given case's details, so that the recorded p'sak of a rav may not be applicable to other cases in which the details are different. Sometimes even the actions of a rav must be considered hora'ot sha'ah, and therefore inapplicable to the world today.
The great care that must be exercised when drawing conclusions from the actions of a rav is stressed by the Talmud itself. The Talmud (Chullin 6b) relates that when Rebbi heard that R. Meir ate fruit grown in the town of Beit She'an without taking t'rumot u-ma'asrot, Rebbi followed this "ma'aseh rav" and ruled that Beit She'an should not be considered part of the Land of Israel (produce grown outside Israel does not require tithing). But he only lent such weight to R. Meir's reported actions because they were attested to by R. Yehoshua ben Zeruz, a torah scholar in his own right, and brother-in-law of R. Meir. Rebbi was sure that R. Yehoshua ben Zeruz would have understood properly what he had seen R. Meir practice, and so he relied on R. Yehoshua's reports in determining the halachah. If R. Meir's actions had been reported by a person of lesser stature, Rebbi would have had to be concerned that important details of the situation had been overlooked, or that R. Meir's actions could only be considered a hora'at sha'ah.
The distinction between mitzvot le-dorot and hora'ot sha'ah is not the only one recognized by halachah; we must also distinguish between mitzvot possessing different levels of holiness. When a yom tov falls on a Sunday, for example, we recite the blessing "ha-mavdil bein kodesh le-kodesh," to distinguish between the higher holiness of Shabbat and the lower holiness of the arriving yom tov.
In fact, there is a hierarchy of importance which encompasses all the mitzvot and distinguishes between different levels of holiness among them. Just as we must know which mitzvot were given le-dorot and which were only intended le-sha'ah, so too we must know today which mitzvot have priority over others, which are the rule and which are the exceptions to the rule.
It is clear that words and ideas take on new meanings when set forth in different arrangements. The philosopher Blaise Pascal, for example, wrote that although each of his individual ideas may have appeared in the writings of earlier philosophers, the arrangement in which he presented them created his unique philosophy (Thoughts 1:22). Similarly, although all poets use words from the dictionary, no one would argue that therefore all poems mean the same thing. The same is true of mitzvot: to interfere with the hierarchy of mitzvot is to change the mitzvot themselves, and to depart from the established values of Judaism.