The Machshava of Halacha
The Gemara tells us:
Eruvin 13b: Rabbi Abba said in the name of Shmuel: Beis Shammai and Beis Hillel argued for three years, each side claiming the Halacha was as they maintained. A bas kol then came forth and stated: Eilu va'eilu divrei Elokim chaim (These and those are the living words of Hashem), but the Halacha is according to Beis Hillel . . .
The Ritva there explains:
The Rabbonim asked how it is possible that these and those are the living words of Hashem when these forbid and those allow. They answered: When Moshe went up to receive the Torah, he was shown in every issue forty-nine manners in which to forbid and forty-nine ways in which to allow. Moshe asked Hashem about this. Hashem told him that the Chachmei Yisroel in every generation, were to decide which manners to follow in their specific times and places . (See also Chagiga 3b; Avos 5:17, and the Maharal in the Derech Chaim there).
The Ritva says that this approach is correct, yet there is a deeper perspective.
Torah Shapes the World
HaGaon of Lublin zt"l writes (Tzidkas HaTzaddik 90): "The Torah is the map of the world . . . and so is Yisroel (since they and the Torah are one. This is because we know that the illumination of Jewish souls is the illumination of the Torah, as it is said that Yisroel is an acrostic: Yesh Shishim Riboh Osios LaTorah [there are 600,000 letters to the Torah]). The Jews in each generation comprise the current map of the world. New phenomena in the Jewish nation in any generation will create corresponding new phenomena in the structure of the world." This idea is not jsut a Chassidic one. HaGaon HaRav Eliyahu Meir Bloch zt"l (Shiurei Da'as, "Darka shel Torah", chap. 5) writes: "When the Torah was given to Yisroel, the characteristics of its nature were imparted to the Torah Sages. They, through their thought, determine the characteristics of nature, which follows the logic and secrets of their Torah. They decide the reality of Torah, and the reality of the Creation linked to the Torah." What is the cause, and what is the effect? The cause is not reality, which demands the effect of figuring out relevant Halachos. On the contrary, the cause is Halacha, and the effect is the reality of the world.1
Let us note Reb Elya Meir's caveat: "If it is in line with the logic and secrets of the Torah . . ." Reb Tzadok elsewhere (ibid., 115) makes a similar remark: "When one is mechadesh a matter in Torah, one must not do so with any negi'a [vested personal interest] in his heart, i.e., that he wants the matter turn out so, for the sake of his pride, or to argue on another, etc. One's chiddush must stem solely from one's yearning to know the truth. If a person follows these guidelines, then even if he makes a mistake his words are words of Torah and divrei Elokim chaim." Not every person under every circumstance can claim to generate divrei Elokim chaim. Only people whose thoughts and conclusions meet these criteria are qualified to create divrei Elokim chaim. If these criteria are met, however, even a mistake (in Talmudic terms, a hava amina) can be considered divrei Elokim chaim!
Why?
How do we understand why Hashem allows two (actually, 49+49=98!) contradictory practical approaches to coexist. Why is the legitimacy of machlokes in Halacha L'Ma'aseh so inherent in Yahadus?
In every Halachic matter there may be conflicting approaches of equal validity. This phenomenon is rooted in the fact that there are distinctions between souls and personalities. Reb Tzadok bases this idea on the Gemara's explanation of the bracha made upon seeing an assembly of 600,000 Jews: Baruch Chacham HaRazim (Berachos 58a), and the description of distinctions between individuals in Sanhedrin 37a and 38a. Hashem's Master Plan specifically required a world of diversity. Am Yisroel is a nation of many different and diverse people. Each member of our nation is created for his or her specific and unique purpose. That is why most of the Torah is Be'al Peh (Gittin 60b). Torah She'biktav corresponds to the entirety of Am Yisroel - and it is therefore static and uniform, as is the eternal kedusha of the Klal of Am Yisroel. Torah She'Be'al Peh, however, corresponds to the particular individuals within the nation. Therefore, just as there are many variations among the individual members of Am Yisroel, there are many variations in Torah She'Be'al Peh.
Reb Tzadok draws an analogy to medicines. Different patients suffering illnesses at distinct times require different - often opposite - Refu'as HaGuf medications. Similarly, different members of Am Yisroel in dissimilar places at distinct times in history require different - often opposite - Refu'as HaNefesh medications. Hashem created a world full of variety and differences. "Different strokes for different folks." The variations in Halacha correspond to the variations among human beings. (A Kabbalistic explanation of these variations along the lines of chesed and gevurah is cited in the Hakdama to Tanya). The inhabitants of the town of Rabbi Eliezer who cut down trees on Shabbos to make coals to forge knives to perform a Bris Mila that day (according to his opinion in Shabbos 130a that machshirei mila are docheh Shabbos) were therefore fulfilling a mitzva and Ratzon Hashem. Their Mara D'Asra, whom Hashem had provided them as a Rofeh HaNefesh, had made such a determination. Inhabitants of any other locality who would engage in the same activity, however, would be liable to capitol punishment!2
Two questions arise: 1) Different centers of Torah learning throughout the generations produced various darkei Avoda and Limud and therefore, different psak halacha. The Hungarian derech differs from the Polish derech, which in turn differ from Lithuanian and Sefardic derachim (see Michtav Me'Eliyahu vol. 4 p. 129). There are wide variations in derech among Poskim of our generation as well. Most of us are not qualified to analyze these derachim and render judgments as to their comparative validity. How then, do we find out who is qualified to state an opinion that may be considered divrei Elokim chayim? 2) If Rabbi Eliezer's opinion (or any other similar opinion) was valid, why can one no longer choose to follow such a psak?
Who?
Obviously, prowess in Lomdus and Halachic methodology is a precondition for acceptance as a Posek. Sometimes smicha recognizes that prowess. More often, haskamos or verbal recognition of universally accepted Gedolei Hora'a validate the positions of aspiring Poskim. Reb Tzadok, however, addresses an additional qualification. Once upon a time Shevet Yissachar (who were "yod'ei bina l'ittim" (Divrei Hayamim 1:12), i.e., they understood what Halachic behavior was suitable for each generation) and Shevet Levi decided what Halachic approach was suitable for whom when (Yuma 26a). Rabbi Yochanan in Chagiga 15b identified their qualification. He explains the pasuk in Malachi: "For the lips of a Kohen guard wisdom and they will seek Torah from his mouth, because he is a malach of Hashem Tzevakos." Said Rabbi Yochanan: "Only if a Rov is like a malach of Hashem Tzevakos may one seek Torah from his mouth." A malach is an agent (a shaliach) of Hashem. An individual who views himself only as an agent of Hashem and focusses on the fulfillment of that agency, is qualified to generate divrei Elokim chayim. The Gemara (Yuma ibid.) explains the description of Dovid HaMelech as "Hashem imo," to mean that Halacha always followed his opinion. Reb Tzadok understands that this is not just a statement of fact. "Hashem imo" was the reason that Halacha was always like Dovid. He fulfilled "Shivisi Hashem l'negdi tamid", and therefore met the criterion of agency that allowed him to be a malach Hashem Tzevakos. Only devoted Talmidei Chachomim who are without negi'os and focussed on detecting Ratzon Hashem, can generate divrei Elokim chaim.3 As we will see, this approach is one understanding of learning lishma.
It is imperative that we note a caveat. I was once asked why the opinion of the Conservative "Rabbinate" that permitted driving to Shul on Shabbos is not considered divrei Elokim chaim. There are, of course, many answers to this question, including the simple fact that their Halachic Decisors do not meet the above criteria. Even where their methodology is erroneous, the fact that they do not fulfill Halacha is not what makes them Non-Orthodox (i.e., Apikorsim, even if, Tinokos She'nishbu). That would only make them Avaryanim, i.e. less or non-observant. It is their denial of Torah min HaShamayim, and several other of the Yud Gimmel Ikkarim that separates them from true Judaism. A "Movement" that denies the principles of Judaism is unacceptable in a way that transcends Halachic methodologies or specific questions of expertise or observance.
Once a Posek is recognized to have attained the above criteria, a layman is not obliged to ascertain the validity of that Posek's Halachic methodology. Hashem helps Poskim to reach legitimate conclusions that are divrei Elokim chaim, and suitable for the Avodas Hashem of the relevant people, places and times. The greatest Poskim became one with the Torah itself, and their capacity to pasken transcended even the Halachic process itself. Once the Chasam Sofer zt"l's son, the Ksav Sofer zt"l, felt that his father's proofs in a certain teshuva were questionable. He asked his father, therefore, about the validity of the resultant psak. The Chasam Sofer responded that in his piskei halacha, the primary factor was his sense of what the psak should be. Specific proofs were secondary in importance (Nefesh HaRav p. 42. See Eitz Chaim p. 430 for a similar statement by HaGaon HaRav Chaim of Volozhin zt"l).
Psak Halacha
We now turn now to the second question: Why may we no longer follow opinions that, like Rabbi Eliezer's, have been rejected? The answer lies in the idea we explored previously, that Torah determines the reality of Creation. When Am Yisroel, via its Poskim and its Minhagim, determines specific issues according to the guidelines that decide psak halacha (i.e., yachid v'rabbim halacha k'rabbim, etc.), that psak shapes the reality of Creation. When subsequent generations approach their responsibility in this world, they face a different set of circumstances than faced by the generation in which the original machlokes occurred. Reb Elya Meir (ibid., chap. 7) explains that this is what Chazal meant when they said (Shabbos 10a): "Every Dayan that judges truly and truthfully is considered by Scripture as a partner with Hashem in the act of creation." The Poskim decide not only Halachic reality, but also the structural reality of the world. As long as the psak is not conclusively decided by Am Yisroel, conflicting opinions may each represent legitimate avenues of practical Avodas Hashem. Once, however, the psak has been decided, the rejected opinion is still Torah, and theoretical divrei Elokim chayim, but it is no longer a legitimate avenue of practical Avodas Hashem. Under the new circumstances of reality, following the rejected opinion might be an aveira. It is easier to make out how various darkei Avoda and Limud correspond on macro-levels. For example, many sources point out that Beis Shammai's trend to chumra corresponded to middas hadin while Beis Hillel's trend to kulla corresponded to middas harachamim. It is more difficult, if not impossible, at least for us, to identify such parallels on micro-levels such as our example, mezuza. In terms of Halachic conduct in this area, however, we have, hopefully, achieved some degree of clarification. No final decision has been rendered in the machlokes over where to place the mezuza. We may, therefore, rest assured that whatever our Rabbonim pasken for us is a legitimate avenue of kiyum mitzvos and Avodas Hashem (although you might like to keep this essay handy to explain why!).
A Broader Perspective
Implicit in the discussion of eilu va'eilu is, obviously, a plug for Ahavas Yisroel. When HaGaon HaRav Isser Zalman Meltzer zt"l first became Rov in Slutzk, an argument broke out in shul whether to say Av HaRachamim on Shabbos Mevarchim Av. The arguing parties asked Reb Isser Zalman to clarify the proper minhag. He said: "This is the minhag: some say Av HaRachamim, some do not, and both sides quarrel about it!"
On a more serious plane, eilu va'eilu teaches us to tolerate others' minhagim and derachim, and to realize that those derachim may also be legitimate avenues of Avodas Hashem. In areas that are the subject of legitimate Halachic debate, there is no one emes, and no justification for personal machlokes, much less, chas ve'shalom, sinas chinam. There is, however, another, more important mussar haskel for us to take away from this discussion. Reb Tzadok (Yisroel Kedoshim 66b) notes that every member of Am Yisroel is rooted in Torah She'Be'al Peh and possesses the unique qualities that it imparts. Although we don't always realize it, each of us contributes a unique quality to the ongoing weave of the rich tapestry of Am Yisroel. Therefore, in a broader sense, each of us is involved in the ongoing creation of Torah She'Be'al Peh. Although most of us are not great scholars and will not produce great works of Torah, with our Torah and mitzvos we all manipulate and shape the Creation (Reb Chaim Volozhiner explains this process in the first section of the Nefesh HaChaim). This quality that Hashem granted us is not just a gift. It is also a tremendous responsibility. As participants in the creation of Torah She'Be'al Peh, we must ensure that are lives are divrei Elokim chayim. That means that we must lead our lives lishma - focussed on discovering and fulfilling Ratzon Hashem.
The Kotzker Rebbe zt"l (Emes v'Emuna p. 26) notes that lishma begins in the way we learn. Torah lishma, said the Kotzker, is the same as Torah kishma. We learn Torah to fulfill the meaning of its name. Torah means "Teaching," and our Torah is Toras Chaim, the Teaching of Life. If we learn Torah with the intent that it elevate and refine our lives, our Torah is lishma, and divrei Elokim chayim. Then, of course, our ma'aseh, our lives themselves must be divrei Elokim chayim.
It is said that HaGaon HaRav Naftali Amsterdam zt"l set a goal to become such a pure tzaddik that everyone he met would want to be a religious Jew (Reb Yaakov p. 29). If we measure our lives by this standard they will truly be divrei Elokim chayim - living words of Hashem.
1.Chazal note that the Sanhedrin determines the reality of the world when they declare a leap year, see Yerushalmi Kesuvos 1:2, and Encyclopedia Talmudis vol. 1 pp. 201-202.
2.It is debatable whether the classic concept of Mara d'Asra still exists. Once, however, local psak determined local reality. HaGaon HaRav Yechiel Michel Gordon zt"l of Lomza related that an indivdual in Volozhin suffered from a certain form of lung disease. The person intended to leave the city and move to a place with better air. The individual's father appeared to him in a dream and told him that his specific form of lung disease was the subject of a machlokes between the Rema and the Sha'agas Aryeh. The Rema held that if this particular form of lung disease occurs in a cow, then the animal is treif, as it is incapable of living for another year. The Sha'agas Aryeh, however, had paskened that an animal with this disease was nonetheless kosher. (The fascinating history of the psak of umma haserucha ladofen im makka badofen is well documented. See, for instance, Makor Baruch chap. 17 section 2.) The father therefore warned his son to remain in Volozhin. His rationale was that in Volozhin, the Sha'agas Aryeh's town, the psak - and therefore the Ratzon Hashem - followed the ruling of the Sha'agas Aryeh. The disease would not threaten this person's life as long as he remained there. Were he, however, to leave Volozhin, he would fall under the ruling of the Rema and would be at mortal risk. (I am indebted to Rabbi Avraham Kivelevitz for finding the source of this ma'aseh in Rabbi Menachem M. Yashar zt"l's essay in the She'eilos U'Teshuvos Sha'agas Aryeh Mahaduras Machon Chasam Sofer note 2.)
3.Reb Tzadok notes that the use of name Tzevakos - "Lord of Hosts" - in this context connotes a variety of individuals. The quality of malach Hashem allows the Rov to understand the proper Refu'as HaNefesh for each different individual. (In Divrei HaYamim 2:36 and Shabbos 119b, Talmidei Chachomim are called "Malachei Elokim." Reb Tzadok understands that description in light of the term Elohim in Parashas Mishpatim that means judges.)
Rab Judah Hindu'a related: Once we were traveling on board a ship when we saw a precious stone that was surrounded by a serpent. A diver descended to bring up the stone. Then the serpent approached to swallow the ship, but a raven came and bit off its head and due to the great size of the serpent the water appeared to turn into blood. A second serpent came, took the stone, placing it on the dead serpent, which brought it back to life. Again the serpent approached the ship, intending to swallow it, but again a bird came and cut off its head. At that point the diver seized the precious stone and threw it into the ship. There were salted birds in the boat, and as soon as the stone was placed on them they came back to life, and taking the stone with them, they flew away with it. 
