Eruvin 36b:3

דְּאִי סָלְקָא דַעְתָּךְ תְּחִילַּת הַיּוֹם קוֹנֶה עֵירוּב, אִי אָמַר עָירְבוּ לִי בָּזֶה, אַמַּאי לֹא אָמַר כְּלוּם? As, if it should enter your mind that an eiruv acquires one’s Shabbat residence at the beginning of the day of Shabbat, then if he said: Establish an eiruv for me with the produce in this flask, why hasn’t he said anything? After nightfall, when Shabbat begins, the flask is already pure, and therefore the teruma of the tithe inside it is also pure and is suitable for an eiruv.

אָמַר רַב פָּפָּא: אֲפִילּוּ תֵּימָא תְּחִילַּת הַיּוֹם קוֹנֶה עֵירוּב, בָּעִינַן סְעוּדָה הָרְאוּיָה מִבְּעוֹד יוֹם, וְלֵיכָּא. Rav Pappa said: This is no proof; even if you say that an eiruv acquires one’s Shabbat residence at the beginning of the day of Shabbat, nonetheless, we require a meal that is fit to be eaten while it is still day, prior to the onset of Shabbat, in order for the eiruv to be valid, and there is none in this case. While it was still day, it was certainly prohibited to consume the contents of the flask, which were still tevel, and therefore it could not be used as an eiruv.

אִם בָּא חָכָם מִן הַמִּזְרָח — עֵירוּבִי לַמִּזְרָח. מִן הַמַּעֲרָב — עֵירוּבִי לַמַּעֲרָב. בָּא לְכָאן וּלְכָאן — לִמְקוֹם שֶׁאֶרְצֶה אֵלֵךְ. לֹא לְכָאן וְלֹא לְכָאן — הֲרֵינִי כִּבְנֵי עִירִי. רַבִּי יְהוּדָה אוֹמֵר: אִם הָיָה אֶחָד מֵהֶן רַבּוֹ — הוֹלֵךְ אֵצֶל רַבּוֹ, וְאִם הָיוּ שְׁנֵיהֶן רַבּוֹתָיו — לִמְקוֹם שֶׁיִּרְצֶה יֵלֵךְ. Similarly, one may say: If a Sage comes from the east and he is spending Shabbat beyond the boundaries of my town, my eiruv is in the east, so that I may go out to greet him there; and if he comes from the west, my eiruv is in the west. If one Sage comes from here, and another Sage comes from there, I will go wherever I wish; and if no Sage comes, neither from here nor from there, I will be like the rest of the inhabitants of my town. Rabbi Yehuda says: If one of the Sages coming from opposite directions was his teacher, he may go only to his teacher, as it is assumed that was his original intention. And if they were both his teachers, so that there is no reason to suppose that he preferred one over the other, he may go wherever he wishes.

גְּמָ׳ כִּי אֲתָא רַבִּי יִצְחָק תָּנֵי אִיפְּכָא כּוּלָּהּ מַתְנִיתִין. קַשְׁיָא גּוֹיִם אַגּוֹיִם, קַשְׁיָא חָכָם אַחָכָם! GEMARA: The Gemara relates that when Rabbi Yitzḥak came from Eretz Yisrael to Babylonia, he taught all of the laws in the mishna in the opposite manner. That is to say, according to him, if the gentiles came from the east, his eiruv would be to the east, and, conversely, if the Sage came from the east, his eiruv would be to the west. This is difficult because if this is correct, there is a contradiction between the ruling concerning gentiles in the mishna and the ruling concerning gentiles in the baraita, and similarly there is a contradiction between the ruling concerning a Sage in the mishna and the ruling concerning a Sage in the baraita.

גּוֹיִם אַגּוֹיִם לָא קַשְׁיָא: הָא — בְּפַרְהַגְבָּנָא, הָא — בְּמָרֵי דְמָתָא. The Gemara answers: The apparent contradiction between the ruling concerning gentiles in the mishna and the ruling concerning gentiles in the baraita is not difficult: This case in the mishna is referring to a tax collector [parhagabena], from whom one wishes to flee; whereas that case in the baraita is referring to the lord of the town, with whom he wishes to speak. Therefore, there are times that one wants to go out toward the gentile, while at other times one wants to flee from him.

חָכָם אַחָכָם לָא קַשְׁיָא: הָא — בְּמוֹתֵיב פִּירְקֵי, הָא — בְּמַקְרֵי שְׁמַע. Similarly, the apparent contradiction between the ruling concerning a Sage in the mishna and the ruling concerning a Sage in the baraita is not difficult: This case in the mishna is referring to a scholar who sits and delivers public Torah lectures, and one wishes to come and learn Torah from him; whereas that case in the baraita is referring to one who teaches children how to recite the Shema, i.e., one who teaches young children how to pray, of whom he has no need. The baraita teaches that if a scholar came from one direction to deliver a public lecture and the school teacher came from the opposite direction, his eiruv is in the direction of the scholar.

רַבִּי יְהוּדָה אוֹמֵר אִם הָיָה אֶחָד מֵהֶן וְכוּ׳. וְרַבָּנַן — זִימְנִין דְּנִיחָא לֵיהּ בְּחַבְרֵיהּ טְפֵי מֵרַבֵּיהּ. We learned in the mishna that Rabbi Yehuda says: If one of the Sages was his teacher, he may go only to his teacher, as we can assume that this was his original intention. The Gemara asks: And what is the reason that the Rabbis do not accept this straightforward argument? The Gemara answers: The Rabbis maintain that sometimes one prefers to meet the Sage who is his colleague rather than the Sage who is his teacher, as sometimes one learns more from his peers than from his teachers.

אָמַר רַב: לֵיתָא לְמַתְנִיתִין מִדְּתָנֵי אַיּוֹ. דְּתָנֵי אַיּוֹ: רַבִּי יְהוּדָה אוֹמֵר אֵין אָדָם מַתְנֶה עַל שְׁנֵי דְבָרִים כְּאֶחָד, אֶלָּא: אִם (כֵּן) בָּא חָכָם לַמִּזְרָח — עֵירוּבוֹ לַמִּזְרָח, וְאִם בָּא חָכָם לַמַּעֲרָב — עֵירוּבוֹ לַמַּעֲרָב, אֲבָל לְכָאן וּלְכָאן — לָא. Rav said: This version of the mishna should not be accepted because of what the Sage Ayo taught to the opposite effect, as Ayo taught the following baraita: Rabbi Yehuda says: A person cannot make conditions about two things at once, i.e., he cannot say that if one Sage comes from one direction and another Sage comes from the other direction, he will go wherever he wishes. Rather, he may say that if a Sage came from the east, his eiruv is in the east, and if a Sage came from the west, his eiruv is in the west. But he may not say that if one Sage came from here, and another Sage came from there, he will go wherever he wishes.

מַאי שְׁנָא לְכָאן וּלְכָאן דְּלָא — דְּאֵין בְּרֵירָה, לַמִּזְרָח לַמַּעֲרָב — נָמֵי אֵין בְּרֵירָה. The Gemara asks: What is different about a case in which one stipulated that if Sages came from here and from there he may go to whichever side he chooses, such that his eiruv is not effective? Apparently, this is due to the principle that there is no retroactive designation, meaning that a doubtful state of affairs cannot be clarified retroactively. However, according to this principle, when one established an eiruv to the east and to the west in order to be able to travel in the direction of one Sage who comes toward the town in a case where one does not know in advance from which direction he will come, we should also invoke the principle that there is no retroactive designation. Therefore, even if one deposited an eiruv at both ends of his town for the sake of one Sage who might come from either side, he should not be able to rely on what becomes clarified afterward and decide retroactively which eiruv he is interested in.

אָמַר רַבִּי יוֹחָנָן: וּכְבָר בָּא חָכָם. Rabbi Yoḥanan said: This is not a true case of retroactive designation, as the Sage had already come by twilight but the person who established the eiruv did not yet know which side of the town the Sage had come toward. Therefore, at the time the eiruv establishes his Shabbat residence it is clear which eiruv the person wants, even though he himself will only become aware of that later.

אַדְּרַבָּה, לֵיתָא לִדְאַיּוֹ מִמַּתְנִיתִין. The Gemara poses a question with regard to Rav’s statement cited above: Why should we reject the mishna because of the baraita? On the contrary, let us say that the ruling of Ayo should not be accepted because of the mishna.

לָא סָלְקָא דַּעְתָּךְ, דְּהָא שָׁמְעִינַן לֵיהּ לְרַבִּי יְהוּדָה דְּלֵית לֵיהּ בְּרֵירָה. דִּתְנַן: הַלּוֹקֵחַ יַיִן מִבֵּין הַכּוּתִים, The Gemara answers: It should not enter your mind to uphold the mishna’s ruling because it contradicts other sources, as we have already heard that Rabbi Yehuda does not accept the principle of retroactive designation. As it was taught in the Tosefta: One who buys wine from among the Samaritans [Kutim], who do not tithe their produce properly,