הלכה על פסחים 103:1

And the truth is that the matter today is only a custom - [and] not [from] any other obligation. As we do not make that day because of a doubt, since all of Israel are now experts in the setting of the months and know the exact day of the appointed time according to the traditional calculation that is in their hand, as I wrote above in the Order of Bo el Pharaoh in Commandment 4 (Sefer HaChinukh 4). And the custom was set because at the beginning when there were ordained men in the Land, they would set the month according to sighting, which is the commandment of the Torah - as we have written there. And therefore all the places of Israel (Jewish settlement) far from the chosen place in which it was fixed there, that messengers could not reach to inform them of the day of the fixing, were in doubt about which day was fixed as the [first of the] month - whether the thirtieth [of the previous month] or the thirty-first. And [so] they would make the appointed time two days from this doubt. However they were never in doubt about more than one day, as they would always fix the new moon on the thirtieth or the thirty-first - whether with witnesses or without witnesses. As the thing is well-known and clear that the new moon never delays more [than this]. And therefore whether witnesses came or did not come, they would set the new month [no later than] the thirty-first. And because of this doubt that there is for those far from Jerusalem, they fixed for them at first to make two days of the holiday (Beitzah 4b). And the Sages, may their memory be blessed, ordained (Pesachim 52a) also now for those far from the Land to make two days like the law that they would do at that time - even though all of Israel have become experts in the fixing of the month, as we said. But it is not fitting for those close and - all the more so - those living in the Land itself that they should do anything but one day, like the custom of the people of the place always. And so have they practiced, according to that which we have heard.

And behold it is seen that our Rabbis in all of the Gemara say the opposite of this. As behold, they determine all rabbinic cases leniently, as they always say (Beitzah 3b), "A doubt of Torah [law] is to be [ruled] stringent and a doubt of rabbinic [law] is to be [ruled] lenient. And they were lenient regarding a concern of rabbinically forbidden [products being present in a certain food], to say, "As I say" (that we can assume that it is not present). And they said in the first chapter of Pesachim 9b, "I will say that we say, 'As I say,' in a rabbinic [law]; in a Torah [law], can we say, 'As I say?'" And they relied upon minors who are not fit to testify, to testify on that which is rabbinic, as they said (Pesachim 4b), "The checking for chamets is rabbinic, and the Rabbis relied upon them in rabbinic [laws]." And so [too,] regarding perimeters, a minor is believed to say, "The perimeter of Shabbat is to here," [as they] "held that perimeters are rabbinic, and the Rabbis were lenient in the rabbinic," as it is found in Eruvin 58b and in Ketuvot 28b. And they were also likewise lenient in the rabbinic with doubts, as they, may their memory be blessed, said (Berakhot 21a), "[If] there is a doubt if he prayed or if he did not pray, he does not go back and pray; [if] there is a doubt if he said, 'True and solid' or he did not say it, he goes back" - and they said, "What is the reason? Prayer is rabbinic; 'True and solid' is from the Torah." And not only that, but they were even lenient about things that contradict each other in rabbinic [law] - as they said in the chapter [entitled] Bemeh Madlikin (Shabbat 34a), "[If] two [people] said to him, 'Go out and make an eruv for us'; he made an eruv for one while it was still day, etc.," as it appears there. And it is also seen in the Gemara that we constantly uproot their words on account of a Torah prohibition; as they said in Tractate Shabbat 4a, "If he stuck bread onto an oven [wall], they permitted him to scrape it off before he comes to a prohibition [that is punished with] stoning." And there in Shabbat 128b they said, "Negating a vessel from its preparedness is rabbinic, but the pain of animals is [from] the Torah, and a positive commandment [from] the Torah comes and negates a positive commandment [from] the Rabbis," as it appears there. And this is [something seen] very much in the Talmud - a positive commandment [from] the Torah comes and pushes off a positive commandment [from] the Rabbis. And so [too] with a disagreement among the Sages, they said (Avodah Zarah 7a), "If one [group] was greater in wisdom, follow it; and if not, follow the stringent [one] in that of the Torah, and the lenient in that of the [Rabbis]." And even greater than this, they said (Eruvin 67b), "In that of the [Rabbis,] we first do the act, and then we deliberate." And in the chapter [entitled], Mi Shehichshikh (Shabbat 154b), they said, "What is it that you would say? They were also concerned with a small loss. Hence, it makes us hear" - as it is a novelty with them when they do not push off the words of the [Rabbis], even for a small loss. And they said (Berakhot 19b) that priests can render themselves impure with rabbinic impurity to see kings of the nations of the world; so that if they merit [it], they will differentiate that [which separates] the kings of Israel, etc. And also regarding the punishments of the words of the [Rabbis], they only have excommunication, as they said (Pesachim 52), "We excommunicate for the two days of holiday in the Diaspora." And they said about one who does work on Purim, "Let the master excommunicate him." And in [some] places, they have lashes of rebellion, and that is for one that transgress their words that are similar to [commandments] of the Torah - and those are all the decrees that they decreed from their [own] words - that they lash him until he accepts it upon him or until his soul departs, as is explained in the Tosefta of Sanhedrin. The general rule of the matter is that the words of the [Rabbis] are different in all of their laws from the words of the Torah, that [the one tends] towards leniency, and [the other] towards stringency. But the thing that is clear and clean of any confusion is that this negative commandment of "you shall not stray" is only in that which they, may their memory be blessed, said in explanation of the Torah - such as things that are expounded through a gezerah shavah or a binyan av or the rest of the thirteen methods through which the Torah is expounded; or about the meaning of the language of the verse itself - and so [too] regarding that which they received as a law of Moshe from Sinai. And it is about this that they, may their memory be blessed, said that there is a positive commandment and a negative commandment in the thing. And if in this matter, one fitting to pronounce decisions disagrees with the Great Court about that which the volitional transgression [brings] excision and the inadvertent violation [brings] a sin-offering, he becomes a rebellious elder through them - at a time when we judge capital cases. And this is [the meaning of] that which they, may their memory be blessed, said (Sifrei Devarim 154), "Even if they tell you about the left that it is right"; meaning to say that this is the commandment upon us from the Master of the Torah, may He be blessed - that we believe the greats regarding what they say, and that the one who disagrees not say, "How can I permit it for myself, since I know with certainty that they are mistaken?" As even if it will be such, it is a commandment to listen to them - as I wrote above at the beginning of the commandment - and like the matter that Rabban Gamliel conducted with Rabbi Yehoshua on Yom Kippur that fell out according to [the latter's] calculation, as is mentioned in Rosh Hashanah 25a.