The Shulchan Aruch, the most accepted code of Jewish law, consists of four sections: Orach Chaim, dealing with the day-to-day routine of Jewish law; Yoreh Deah, dealing with Jewish ritual law, primarily that of kashrut; Choshen Mishpat, monetary law; and Even HaEzer, family law. While there are only four sections in print, it has often been noted that there is a fifth section to the Shulchan Aruch, one no less important than the other four—that of common sense.
Halachic decisions are not to be made by a robot or a computer (or even Rabbi Google), but by one in possession of great common sense. The Talmud has an expression, "Lamah li kra, sevara huh?, why do we need a Biblical verse, [we deduce it] from logical reasoning?" (see for example, Bava Kamma 46b). So crucial is basic common sense that our rabbis teach that “any Talmud chacham, Torah scholar, who does not possess da’at, common sense—beheimah neveilah, a dead carcass is better than he” (Vayikra Rabba 1:15). Sadly, and tragically, one can be a great Torah scholar and a fool at the same time.
Yet common sense has it limitations—especially when dealing with disciplines that require a high degree of knowledge. In a famous derasha dubbed “The Common Sense Revolution”, Rav Soloveitchik explains that common sense was the rallying cry of Korach. Arguing that a house full of books should not need a mezuzah or that a shirt of techelet should not need a tzitzit string of the same, Korach suggested that Moshe lacked common sense and thus, a new leader was needed. The halachic system, like all legal systems—and all specialized disciplines, for that matter—has its own internal logic that to an outsider may seem strange, even defying common sense, but is central to the proper application of the system at hand.
For example, in our legal system it is possible to be acquitted in criminal court of a murder yet found guilty in civil court for damages caused by that murder. On a purely logical plane, this makes no sense. One either killed the person or not. One cannot be legally obligated to pay damages for a murder that the same legal system says one did not commit. Yet we all understand that there are very good and necessary reasons for the rules of evidence to differ regarding criminal law and civil law. Common sense must, at times, give way to a self-contained system, operating with its own internal logic. Yet all too often, it is not clear exactly where that line between common sense and the systematic rules should be drawn.
Recognizing the tension between the two adds a layer of meaning to Hillel’s rise to become Nasi, official leader, of the Jewish people.
It was an erev Pesach that fell on Shabbat—a rare occurrence (and one we will experience in some six-weeks’ time)—and the Bnei Beteirah could not recall if the korban pesach was to be brought on Shabbat. Unashamed to admit they did not know, they asked if anyone might know, perhaps an elderly person who recalled what happened the last time erev Pesach was on Shabbat. When no one knew the answer, it was suggested they ask the new rabbi in town. “There is a certain man who ascended from Bavel, and Hillel the Babylonian is his name. He served the two gedolei hador, the most eminent scholars, of the generation, Shemaya and Avtalyon, and he certainly knows whether the Paschal lamb overrides Shabbat or not. They sent and called for him” (Pesachim 68a). Hillel then gave two arguments as to why the korban pesach may be brought on Shabbat.
The Bnei Beteirah, in an act of great humility almost unheard of, “immediately seated him at the head and appointed him Nasi over them”.
Hillel’s first argument was based on a kal vachomer—literally “light and heavy”, an a fortiori argument—wherein if a law applies in one case, the kal, how much more so must it apply in a second more serious case, the chomer. “If the daily offering, the neglect of which is not punishable by karet, excisioin, overrides Shabbat, how much more so the Paschal lamb, the neglect of which is punishable by karet, should override Shabbat?”
Hillel then put forth an argument based on a gezeirah shava, a literary technique wherein when the same word is used in two different sections of the Torah, a law that applies in one section is then applied to the other. “B’moado, ‘its appointed time,’ is stated with regard to the Paschal lamb, and 'b'moado', is stated with regard to the daily offering. Just as ‘b'moado’ which is stated with regard to the daily offering, indicates that it overrides Shabbat, so, too, ‘b'moado’ which is stated with regard to the Paschal lamb, indicates that it overrides Shabbat.”
Kal vachomer and gezeirah shava are the first two of Rabbi Yishmael’s 13 hermeneutical principles used to read “between the lines” of the Torah to discover new layers of meaning. While they are both interpretive tools, it is hard to think of two more distinct ways to explain a text. They are the antithesis of one another.
A kal vachomer is an argument based on logic] and nothing else. It is what we might call the common-sense approach to Torah. No Biblical verses are cited, no precedents are referred to, no rabbinic authority offers support. Yet the correct logical approach can be a matter of debate. Interestingly, while Hillel was appointed the Nasi immediately after presenting his kal vachomer, a few lines later, the Talmud rejects the kal vachomer of Hillel. It presents a counterargument, noting that the reason the korban tamid is brought on Shabbat is because it is tamid, constant, and is brought each and every day. Furthermore, the korban tamid is completely sacrificed on the altar with no consumption by humans, whereas the korban pesach is brought to be eaten. It is the korban tamid and not the korban pesach which is the chamor, and hence the fact that the daily offering overrides Shabbat offers no evidence regarding the korban pesach and Shabbat. What constitutes the kal and what constitutes the chomer is not always simple to determine. But the only way to determine such is by logical inference.
A gezeirah shava, on the other hand, has no inherent logical basis. The fact that the same word appears in two different contexts may be significant but is just as likely to be of no significance. The potential for misuse of this tool is so great that our Sages declared that, “One may not expound a gezeirah shava on one’s own” but must have a received tradition in order to do so. Why an extra word in this context is meaningful, but not in another, has little internal logic. It is part of a divine system that defies simple logic and cannot be assessed through logical intuition.
Our halachic tradition rests on both kal vachomer and gezeirah shava, logical inferences and fixed statutes, common sense and divine fiat. There may be tension between them, but both are integral to our tradition and Hillel invoked them both to explain why the korban pesach, the founding ritual of the Jewish people, can be brought on Shabbat. Leadership requires exceptional logical reasoning and exceptional understanding of its limits—both kal vachomer and gezeirah shava. Who greater to appoint as the leader of the Jewish people than one who can use both to reach the same conclusion?
 In the halachic system, this concept is known palginan diborah, wherein we literally “split the words”, both accepting and rejecting what one says. Thus, if Reuven testifies that Ploni (John Doe) had an affair with Reuven’s wife, if another witness comes forward to corroborate Reuven’s testimony, we would (in theory at least) put Ploni to death but spare the wife (Sanhedrin 10a). This, because we cannot accept testimony regarding relatives, so that legally, there are two witnesses confirming that Ploni commited adultery, but only one that the wife did so.
 While one can distinguish between logic and common sense, I believe that, at least for purposes of our discussion, it is a distinction without a difference.
 This literary approach has become quite popular today in the study of Tanach, especially in Israel, opening up new and exciting horizons in the revival of the study of Tanach. However, there is a big difference as to whether one uses such a method for beautiful textual insights or to derive practical halacha.
 As every halacha has an ethical and/or philosophical corollary, once a gezeirah shava is established, we must look for a deeper connection. For example, our Sages derive that one can marry by gifting something of monetary value to a woman (the Torah itself speaks only of sexual relations effecting marriage) from the use of similar wording in the case of marriage and the purchase by Avraham of a burial plot for Sarah. As Rav Shimshon Raphael Hirsch explains, when one marries, one must care for one’s wife even beyond “’til death do us part”.