Starting premise: Chazal were not fond of people asking questions solely to waste their time. We see this from, for instance, Bava Basra 23b:

בעי ר' ירמיה רגלו אחת בתוך חמשים אמה ורגלו אחת חוץ מחמשים אמה מהו ועל דא אפקוהו לרבי ירמיה מבי מדרשא

R' Yirmiya asked, "If one leg was within fifty amos and one leg beyond fifty amos, what is the halacha?" For this they expelled R' Yirmiya from the Beis Midrash.

Critically, Tosfos there note that the issue with R' Yirmiya's question had nothing to do with its improbability, but rather the fact that he was questioning a basic premise of how measurements work that showed he was just wasting their time:

ועל דא אפקוהו לרבי ירמיה מבי מדרשא. אין לפרש משום דבעי מילתא דלא שכיחא כלל דהיכי איתרמי דרגלו אחת תוך חמשים ורגלו אחת חוץ לחמשים בצמצום דהא אשכחנא דמתניתין נמי בכהאי גוונא איירי דקתני מחצה על מחצה יחלוקו ונראה לרבינו תם דמשום הכי אפקוהו משום דמדדה אינו מדדה כלל יותר מחמשים אמה אפילו רגלו אחת דכל מדות חכמים כן הוא

"For this they expelled R' Yirmiya from the Beis Midrash" – Don't explain that it's because he asked a question which never occurs, as how can it be so precise that one leg is within fifty amos and one leg is beyond fifty amos, because we find that our Mishnah as well discusses a similar case, as it teaches, "Half and half – they split." It appears to Rabbeinu Tam that because of this did they expel him: because the measurement is not a measurement at all beyond fifty amos, even if it's just one foot, as all measurements of the Chachamim are like this [i.e. this precise].

(If I may add to Tosfos, this isn't the first time R' Yirmiya's questioned this premise; cf. Rosh Hashanah 13a.)

It's implicit in Tosfos that the issue is specifically wasting time; were it simply an impossible question, there would have been no problem with it.

Yet we find in Menachos 37a that Rebbi threatened to exile or excommunicate someone for asking an impossible question until he found that question was not only possible but had actually happened:

בעא מיניה פלימו מרבי מי שיש לו שני ראשים באיזה מהן מניח תפילין א"ל או קום גלי או קבל עלך שמתא אדהכי אתא ההוא גברא א"ל איתיליד לי ינוקא דאית ליה תרי רישי כמה בעינן למיתב לכהן

Plimo asked of Rebbi, "Someone who has two heads – on which one should he place Tefillin?" He said to him, "Either go into exile or accept upon yourself excommunication." At that time, a man came and asked him, "A child was born to me with two heads – how much do I need to give to the Kohen [for Pidyon HaBen]?"

Rashi clarifies Rebbi's complaint against Plimo:

או עמוד וצא בגלות או קבל עליך שמתא דאחוכי חייכת בי

Either get up and go into exile or accept upon yourself excommunication, because you are mocking me.

If impossible questions are okay, what was it about Plimo's question which Rebbi felt was mockery? And if impossible questions are not okay, then what of Tosfos' proof that that's not why R' Yirmiya was expelled?

  • 1
    Isn't Tosfos in Bava Basra saying that it's not an impossible question?
    – Heshy
    Commented Mar 3, 2020 at 16:45
  • @Heshy In which part? Tosfos' first statement seems to be saying that it is an impossible question but we don't care, but perhaps Rabbeinu Tam can be interpreted as it's a possible scenario but it shows a basic misunderstanding of Shiurim.
    – DonielF
    Commented Mar 3, 2020 at 22:19

2 Answers 2


There is a difference between questions which are theoretically impossible or which simply don't happen, and things which are naturally impossible. Halacha at its core is a theoretical framework which we apply practically, so we should be fine with purely theoretical discussion, as long as it fits with the natural framework that we live in. We would not accept theoretical questions about Martians for example (can they convert to Judaism?), because that has no meaning in our reality, so it has no place in Halachic discussion. If two-headed people naturally cannot exist, then asking about them is like asking about Martians, and it shows a lack of understanding of the Halachic reality.

  • 1
    – DonielF
    Commented Mar 3, 2020 at 22:17
  • I disagree. What's wrong with asking how halacha, as derived from our actual reality, would handle theoretical cases, whether they actually exist or not?
    – DonielF
    Commented Mar 3, 2020 at 22:18
  • @DonielF Halacha is expected to fit within reality. We can ask about extreme/unlikely/virtually impossible cases as a way to refine the Halachic boundaries only so long as the case is consistent with our observed reality. As soon as we leave this reality the question becomes meaningless, as it cannot help us refine the definitions, and cannot give us any practical guidance.
    – simyou
    Commented Mar 4, 2020 at 6:08
  • Can you imagine Chazal trying to field questions about electricity based on your premise? That's certainly within our reality. What's wrong with asking about something which theoretically could be in our reality but isn't necessarily?
    – DonielF
    Commented Apr 24, 2020 at 21:54

I think Rebbi was simply not aware of the fact that some people are born with two heads, so he felt he was being mocked. But some questions are angrily rejected because they seem to abrogate a law of the Torah, even though the reasoning is sound and follows Rabbi Yishmael's 13 principles of hermeneutics. Here is a case of rejection of a sound kal vachomer (a fortiori) argument for that reason:

Rabbi Yosei ben Taddai of Tiberias said to Rabban Gamliel: My daughter is forbidden to me, but her mother [my wife] is permitted to me. If the daughter of someone who is permitted to me is NOT permitted to me, then all the more reason why the daughter of someone who is NOT permitted to me should NOT be permitted to me. Now, most mothers are married and are forbidden to me. If they are forbidden to me, then so are their daughters. Therefore all marriages are forbidden.

Rabban Gamliel promptly placed him under cherem (excommunicated him). [Derech Eretz Rabbah, chapter 1]

  • For some reason the last 3 lines of the quote made me chuckle.
    – user16556
    Commented Mar 5, 2020 at 19:15

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