Night and day reflect polar opposites. The former symbolizes hope and excitement, the latter fear and trembling. They join together to form a complete day, but separate they must remain. Hence, those mitzvot that are to be done in the daytime, such as shofar, tzitzit, hallel, or lulav, can be performed during the day only. And those mitzvot that are to be performed at night, such as harvesting and counting of the omer or eating matzah, may be done only at night.
Included among daytime mitzvot is the offering of korbanot. While the korban itself can be brought during the day only, the fat and bones of the animal sacrifices were left on the altar to burn through the night. The Gemara was uncertain whether the minchat nesachim, the flour offering accompanying the daily korban tamid, had the same status as the korban itself and hence, also had to be brought in the daytime; or if it was akin to the subsidiary of the korban, which could be brought at night.
“Rav Yirmeya [was] sitting and saying in the name of Rabbi Yehoshua ben Levi: From where is it derived that the nesachim, libations that come with an offering, may be sacrificed only in the day? It says, ‘your libations, and your peace offerings’ (Bamidbar 29:39): just as peace offerings are [brought] in the day, so, too, libations [are brought] only during the day” (Temurah 14a).
The above teaching, based on a biblical text, would seem to refute the view of Rav Yosef who, lacking a biblical source, argued that minchat nesachim may be brought at night. And Rav Yosef retracted his view. Nonetheless, when Rav Dimi heard this teaching, he exclaimed, “If I could have found [a messenger], I would have sent a letter to Rav Yosef” explaining that there is no need for him to retract his view. Only those menachot accompanying an animal sacrifice must be brought during the day. However, menachot nesachim brought independently may be offered at night.
The Gemara is astounded by this discussion! Not because of the distinction Rav Dimi makes between the various types of korbanot. Rather, the Gemara is astonished that Rav Dimi would even consider sending such a letter. Doing so would be a violation of the prohibition of writing down the Oral Law, a prohibition rooted in the Torah itself.
“Rav Yehuda the son of Nachmani—the interpreter of Reish Lakish—expounded: One verse says, ‘Write down these words’ and one (actually the same) verse says, ‘Ki al pi, because with the saying of these words I established a covenant with you. This [verse] comes to teach that oral words may not be said from a written text and the written words may not be said orally” (Temurah 14b).
This is a most serious prohibition. “Rav Yochanan said: Those who write halakhot, it is as if they burn the Torah”.
G-d established two covenants with the Jewish people—two parallel but separate covenants—and never the twain shall meet. The written covenant is the word of G-d; permanent, unchangeable and in many ways unknowable. The Oral Law is the word of man; fleeting, flexible, and subject to re-interpretation. Between man and G-d is an unbridgeable gap—one that we can narrow, but not close, through the study of Torah and performance of mitzvot. The Written and Oral need each other, but they must remain separate domains.
And yet, despite reflecting the understanding of man, it is the Oral Law that reigns supreme—even, or shall we say, especially when it “contradicts” the Written Law. “Rav Yochanan said: G-d established His covenant with Israel only because of the oral words” (Gittin 60b). While the Torah writes “an eye for an eye”, we follow the Oral Law and pay monetary compensation instead. And lest one think that monetary compensation atones for blinding someone, the Written Law reminds us that saying so is no less than a distortion of Torah.
Mixing up the Written and Oral Torah is no less that a nullification of Torah. But nullify the Torah we must. “’There is a time to act for the Lord, they nullified Your Torah’; it is better that the Torah be uprooted than it be forgotten from Israel” (Temurah 14b). Yes, we are prohibited from writing down the Oral Law, but it is an even greater prohibition to allow Torah to be forgotten.
 The only exception to this rule is kriat shema, the acceptance of G-d and His mitzvot. This must be done day and night. The reading of the Megillah at night is a latter-day custom, not even mentioned in the Mishna.
 During Talmudic times, it was common for the heads of yeshivot to have “interpreters” who would review, explain and expound on the contents of their shiurim. Most famous was Rav Chutzpit Hameturgaman, who served in this role for Rabban Gamliel and whose tragic death we will read about on Tisha B’Av as we mourn the asara harugei malchut. This is a tradition that continues in modern times, allowing many students to greatly enhance their understanding of the lectures of great, deep thinkers; and this was something from which I greatly benefited when I was in the shiur of Rav Aharon Lichtenstein, zt”l.
 While we generally translate the word ki as “because”, the Gemara notes that the word ki has at least four meanings: because, when, if and despite. Thus, the verse can (and should) be read to say, “Write down the words of the covenant ki, despite, the fact that the covenant is established through al pi, the Oral Law”.
 If my understanding is correct (and it may not be, even though I am over 40), Kabbalistic thought developed the notion that man and G-d need each other.
 In his fascinating introduction to the development of the Oral Law, Rav Moshe Shmuel Glassner, in his Dor Revi’i, The Fourth Generation (he was the great-grandson of the Chatam Sofer), explains that it is precisely because the written word is permanent and oral is not that the bulk of Torah was meant to be oral. G-d wanted each generation to apply the eternal principles of Torah to its own particular set of circumstances and thus, Torah had to be oral. Precedent was to play a very limited role. It was exile that caused us to move away from this ideal and forced the writing down of Torah, thereby constricting it. Rav Glassner, who made aliyah from Hungary in 1922, saw the return to the land as the first step in returning Torah to its original oral form. (See here for the original Hebrew introduction and here for a synopsis in English.)