The concepts of grue and bleen depend on our color concepts. However, we could consider grue and bleen to be fundamental, and define our color concepts based on them! An object is defined to be green if it is grue before the year 2010 and bleen during or after the year 2010. An object is blue if it is bleen before the year 2010 and grue during or after the year 2010.
Neither of these sets of definitions can be shown to be formally more correct than the other, but only one of them accords with our common-sense understanding of the real world.
And that's how it seems on Megillah 2b, where R. Yehoshua ben Korchah (in a baraita) uses green and blue, and the anonymous tanna of the Mishnah uses grue and bleen.
It is well-known that walled cities (such as Jerusalem) observe Purim on 15 Adar, one day later than unwalled towns. You might have thought that this was because Shushan took an extra day to kill all the bad guys and observed the very first Purim one day late on 15 Adar (Esther 9:18), and Shushan was a walled city, so it became the paradigm for other walled cities in perpetuity. This explanation makes a lot of sense. And R. Yehoshua ben Korchah would agree with you. He says that Purim is observed on 15 Adar in cities that were walled at the time of Achashverosh, by analogy with Shushan.
But the Mishnah (or its stammaitic avatar) would tell you that you haven't read Esther closely enough! Dig:
[Esther 9:17]That was on the 13th day of the month of Adar; and they rested on the 14th day and made it a day of feasting and merrymaking.  But the Jews in Shushan mustered on both the 13th and 14th days, and so rested on the 15th, and made it a day of feasting and merrymaking.  That is why village Jews, who live in unwalled towns, observe the 14th day of the month of Adar and make it a day of merrymaking and feasting, and as a holiday and an occasion for sending gifts to one another.
Full stop. In summary: The first Purim was on the 14th outside Shushan and on the 15th in Shushan; Purim today is on the 14th in unwalled towns. But nothing is said about what is done today in walled cities! (We were joking about Purim Mitzrayim vs. Purim dorot, as in Pesachim 9:5.) Now you or I or R. Yehoshua ben Korchah or any IQ test for children would make the obvious logical leap and say that Purim is observed on the 15th in walled cities. But the mainstream voice of the Gemara (representing the Mishnah's position) has to make it more complicated.
I guess someone has to stand up for the "wacko school of Akiva". So, as discussed last time, the rabbis derive the laws of a "rabbinic" holiday from scripture, viz. a part of scripture that no one claims is the word of God, by using all of the standard textual techniques for deriving "d'oraita" laws from the Torah: gezeirot shavot; finding deep significance in individual letters; the principle that a bijection exists between the set of usable elements (verses, phrases, whatever) in scripture and the set of laws that can be derived from them. It makes you wonder what they thought they were doing. Did they really think they could derive halacha this way from the book of Esther? (It's not out of the question -- Purim is traced to the time of the late prophets, so it has a firmer foundation than laws enacted by the tannaim or amoraim themselves.) Or is this asmachta b'al'ma, finding support in the text for laws whose origin is elsewhere? (I mean, of course it is, but let's play the game.) Or, given the subject matter, is this the first example of Purim Torah (like the Simpsons Talmud)?
So according to the stam representing the Mishnah, there is a gezeirah shavah of p'razi p'razi (unwalled towns) between Esther 9:19 and Deuteronomy 3:5. The latter is talking about the conquest of walled and unwalled cities in Og's kingdom of Bashan, at the time of the original conquest of Eretz Yisrael. From this gezeirah shavah, we see that the definition of p'razi is "cities that were unwalled at the time of Joshua [and bleen after the year 2010]". Esther 9:19 says that p'razim observe Purim on 14 Adar, so we get the Mishnah's rule that cities unwalled at the time of Joshua observe Purim on 14 Adar. By creative parsing of bizmaneihem (Esther 9:31) we know that there exists a distinct set of locales that observe Purim on 15 Adar. Thus, taking the complement of p'razim, we find that Purim is observed on the 15th in cities that were walled at the time of Joshua. Voila!
There's just one problem. Shushan itself was unwalled at the time of Joshua!!! And we know that Shushan observed Purim on the 15th! Well, uh, (according to Rava, or maybe Kedi) Shushan is different, because that's where the miracle happened. So Shushan observes Purim on the 15th, unlike all of its fellow p'razim.
Thus Shushan, which we thought was the whole basis for this unwalled/walled distinction, becomes irrelevant to the general rule, and just an inconvenient special exception. Rosh pinah hay'tah l'even ma'asu habonim.
The superscript letter indicates that the Mishnah's rule, crazy as it is, gets codified. However, the Rambam (Hilchot Megillah 1:5) gives a completely different reason: the walled/unwalled distinction is based on the time of Joshua in order to honor the land of Israel, which was in ruins at the time of Achashverosh but had walled cities at the time of Joshua. Thus, these cities in Israel get to claim their honored status as walled cities, which they wouldn't be able to do under R. Yehoshua ben Korchah's rule. This reason apparently comes from the Yerushalmi, which unsurprisingly wants to honor the land of Israel, while the Bavli has to discover a gezeirah shavah to explain the Mishnah.
On with the Gemara. Neither the Mishnah nor RYbZ needs "ve'ir va'ir"(Esther 9:28) to make his/its point, so by the principle of bijection, it's a sitting duck for interpretation. So this is where we'll staple the statement of R. Yehoshua ben Levi: the walled city and everything connected to it and everything seen as part of it are considered part of the walled city. This is the basis for the contemporary practice of observing Purim on "Shushan Purim" in the modern city of Jerusalem, most of which is outside the walls (and outside any part of the city that was ever walled). R. Yirmiya (or was it R. Chiya bar Aba?) specifies the distance: 1 mil (~1 km), defined as the distance from Chamtan to Tiberias. But what's up with that? Parts of modern Jerusalem are certainly more than 1 mil from the Old City and still observe Purim on the 15th. What is the definition in use these days? Is it the municipal city limits of Jerusalem as determined by the government? Are there neighborhoods that are legally part of Jerusalem that observe the 14th, or (less likely, since the borders of today's Jerusalem sprawl so far) places outside the city limits that observe the 15th?
The Gemara then opens up the FBI file on R. Yirmiya (or was it R. Chiya bar Aba?), listing a bunch of things that he (or was it he?) said. But it's not just mnemotechnical randomness -- all of these meimras (meimraya?) deal with meta issues regarding the text of Tanakh, in the original or in translation. This is an appropriate topic for Masechet Megillah, even if the Mishnah doesn't get there for a few chapters.
The special "final" forms of some of the Hebrew letters come from the prophets! But wait, the final mem must have existed at Sinai, because we have this tradition that the letters on the tablets were carved all the way through, and the inner parts of the final mem and the samech were floating there magically (this is why those Hebrew stencils had a connector piece for those letters -- as we'll see in a few pages, la b'chol sha'ta v'sha'ta mitracheish nisa, a miracle doesn't happen all the time), so we know that the final mem (like the samech) included a closed curve. Hmm. Ok, the final forms existed already, but then they were forgotten, and the prophets brought them back.
Putting aside the problem of the tablets being written in k'tav ashuri rather than k'tav 'ivri, does anyone know whether this bears any resemblance to the actual history of the Hebrew alphabet? Without the prophet part, I mean. I have a vague memory of hearing that the final forms of those 5 letters were actually the original forms, and the non-final forms came later.
Onkelos translated the Torah into Aramaic from the tradition of R. Eliezer and R. Yehoshua, and Yonatan ben Uzziel translated the Prophets from the tradition of Haggai, Zechariah, and Malachi (prophets themselves).
We already know that Yonatan ben Uzziel had superpowers. On Sukkah 28a we learned that Hillel the Elder had 80 students, and the least of them was Rabban Yochanan ben Zakkai, and the greatest of them was Yonatan ben Uzziel. How great was he? When he was sitting engaged in Torah and a bird flew overhead, it immediately burst into flames.
Here on Megillah 3a we see another instance of his superpowers. When he translated the Prophets, the land of Israel shook 400 parsa by 400 parsa (~1000 miles x 1000 miles; I didn't realize Israel was that big!), and a bat kol came out and said "WHO HAS REVEALED MY SECRETS TO HUMANS?". Yonatan stood on his feet and said "You know that I didn't do it for my own glory but for yours, so that disputes would not multiply in Israel." (That worked out well.) He asked the bat kol "While you're here, could you reveal the translation of Ketuvim?" The bat kol said "You've had enough!" And to this day, there is no official targum of Ketuvim. Why not? Because it reveals the time of the messiah.
The Gemara asks a good question: why didn't the land of Israel shake when the translation of the Torah was revealed too? The answer: the Oral Torah had already been revealed with the Written Torah, so Onkelos's translation contained no new content. In contrast, there was no "Oral Neviim" along with the "Written Neviim", so Yonatan's translation was the first time the "Oral Neviim" was revealed. This revelation was what made Israel tremble.
We noted that there was thunder and lightning and all that when the Oral Torah was revealed -- not when Onkelos translated it, but at Sinai! And we can expect similar cataclysms when the Oral Ketuvim is finally revealed -- when the messianic age arrives!
(Though if there's no Oral Ketuvim, then what the heck were we doing on the previous amud? See the first half of this post.)
Finally, let's look at Daniel 10:7. "I, Daniel, alone saw the vision; the men who were with me did not see the vision, yet they were seized with a great terror and fled into hiding." Who were these men? Haggai, Zechariah, and Malachi. They were greater than Daniel in that they were prophets, but he was greater than they in that he saw this vision. (Then in what way were they prophets that he wasn't? Rashi says it's that they were sent by God to prophesy to Israel and Daniel wasn't.) The big question: If they didn't see the vision, then what were they afraid of? Answer: Their mazal (constellation, or as Rashi says, their guardian angel) saw it.
Ravina generalizes this: Any time you are terrified without seeing something terrifying, it's because your mazal (some emanation of you outside your body) saw it. But the lawyers will point out that if there is a fright then there must be a remedy. The remedy: say Shema! If you're in an unclean place, get out. And if that doesn't work, say "The goat in the butcher's house is fatter than I am." As the Jastrow jackpot helpfully explains: "(a charm)"