How would Halacha deal with the Ship of Theseus thought experiment?

For example, if one made a neder (vow) not to have any hana'ah (benefit) from a certain ship and over time every single piece of that ship was replaced, would it still be forbidden? And if someone built a second ship out of all of the pieces from the first, would that one be forbidden?

Similarly, if one signed a contract with someone to sell them a certain axe in two years (should it still be in existence at that point), but during those two years both the handle and the blade were replaced, would that be halachically the same axe?

There are many other cases where this could be relevant halachically, but I think you get the point. Have any Rabbanim ever discussed such a question? Any sources or proofs would be appreciated.


4 Answers 4


This is known in Talmudic literature as panim chadashot ba'u lekan (lit. "a new face has come here"), and is the topic of discussion in Eruvin 24a.

The upshot of the discussion is that, at least as far as the rules of tumah vetaharah and mechitzot are concerned, once all the original components have been replaced, we view the object as something new. (For those interested, I brought the full quote of the relevant piece of gemara below the line.)

It's worth noting (as mentioned by N.T. in a comment) that in the particular examples which you mention in your question - nedarim and contracts - we tend to follow normal usage of language, as is presumed to be understood by the relevant parties. That being the case, it's likely that the halachic analysis of this philosophical question is less relevant here; instead we would simply wish to know how most people speak/think, which would allow us to resolve the question of intent.

אמר רב נחמן אמר שמואל: קרפף יותר מבית סאתים שלא הוקף לדירה, כיצד הוא עושה? פורץ בו פירצה יותר מעשר, וגודרו ומעמידו על עשר, ומותר. איבעיא להו: פרץ אמה וגדר אמה [ופרץ אמה וגדרה], עד שהשלימו ליותר מעשר, מהו? אמר ליה, לאו היינו דתנן: כל כלי בעלי בתים, שיעורן כרמונים. ובעי חזקיה: ניקב כמוציא זית וסתמו, וחזר וניקב כמוציא זית וסתמו, עד שהשלימו למוציא רמון מהו? ואמר ליה רבי יוחנן: רבי, שניתה לנו: סנדל שנפסקה אחת מאזניו ותיקנה — טמא מדרס. נפסקה שניה ותיקנה — טהור בה מן המדרס, אבל טמא מגע מדרס. ואמרת עלה: מאי שנא ראשונה — דהא קיימא שניה? שניה נמי — הא קיימא ראשונה?! ואמרת לן עלה: פנים חדשות באו לכאן. הכא נמי, פנים חדשות באו לכאן. טוקרי עליה: לית דין בר אינש. איכא דאמרי: כגון דין בר נש.‏

R. Nahman laid down in the name of Samuel: If a karpaf that was bigger than two beth se'ah was not originally enclosed for dwelling purposes, how is one to proceed? A breach wider than ten [cubits] is made in the surrounding fence, and this is fenced up so as to reduce it to ten cubits and [then the movement of objects] is permitted.

The question was raised: What is the ruling where one cubit [width of fence] was broken down and the same cubit [of breach] was fenced up and [then the next] cubit [width of fence] was broken down and was equally fenced up [and so on] until [the breaking down and the re-fencing] of more than ten [cubits width of the fence] was completed? — [This case], came the reply, is exactly [the same in principle as the one about] which we learned: All [levitically defiled wooden] utensils of householders [become clean if they contain holes] of the size of pomegranates; and when Hezekiah asked: ‘What is the ruling where one made a hole of the size of an olive and stopped it up and then made another hole of the size of an olive and stopped it up [and so on] until one completed [a hole] of the size of a pomegranate?’ R. Johanan replied: Master, you have taught us [the case of] a sandal, for we learned: ‘A sandal one of the straps of which was torn off and repaired retains its midras defilement. If the second strap was torn off and repaired [the sandal] becomes free from the midras defilement but is unclean [on account of its] contact with midras’. And you asked in connection with this, ‘Why is it [that the absence of the] first [strap does not affect the status of the sandal? Obviously] because the second strap was then available [but then the absence of the] second strap also [should not affect the status of the sandal] since the first was then available?’ And then you explained this to us [that ‘in the latter case] the object had assumed a new appearance; well, in this case also [it may be explained that] the object had assumed a new appearance; [and Hezekiah] made concerning him the following remark: ‘This [scholar] is no [ordinary] man’ [or as] some say: ‘Such [a scholar] is [the true type of] man’.

(Soncino translation, my emphases)

  • 1
    That may be, but nedarim usually go by lashon b'nei adam, so my guess would be it depends on what the speaker is presumed to mean.
    – N.T.
    Jun 14, 2021 at 10:51
  • @N.T. That's true (and maybe I'll add it in as a note to my answer). I assumed that the OP was only using nedarim as an example, and was looking for any halachic discussions that grapple with this philosophical concept.
    – Joel K
    Jun 14, 2021 at 10:59
  • 1
    See also judaism.stackexchange.com/a/119556/4940
    – magicker72
    Jun 14, 2021 at 11:27

Rabbi J.D. Bleich has written an article on this issue titled "The Problem of Identity in Rashi, Rambam, and the Tosafists," available here.

Mark Steiner mentions this issue in passing in his article "Rabbi Israel Salanter as a Jewish Philosopher," available here:

For a simple example, consider the ancient Greek debate about change and becoming. Opinions on the subject ranged from that of Heraclitus, who held that one cannot step into the same river twice (because new waters are always flowing upon you), to Parmenides who held that change is an illusion even in the case of a river. The question crucially depends upon the concept of identity through time—whether we can think of a river as “the same,” as persisting through time, even though the waters coursing through it may be “different.” Now the Talmud asked almost the very same question, though camouflaging it in legal terminology: if one worships (prostrates himself before) a spring, are the waters of that spring made unfit to be offered as a libation? “Is it the water before him that he worshipped, which is no longer there, or is it the [entire] stream of water that he worshipped?” (Avodah Zarah, 48b). The question is not simply one of the idolater’s intention, since there is no way to determine that. Rather the question is philosophical, whether or not the spring itself is an object which persists through time, and therefore could be the object of worship. We have here the beginning of an ontological discussion, despite the ostensible hostility of the Talmud to “philosophy” or “Greek wisdom.” Questions of identity permeate the halakhic literature. The Rabbis had to decide when and whether two idols are the same deity; and at what point a sandal, undergoing alterations, becomes a different sandal. Identity questions can decide even questions of kashrut.

Note 12 reads:

The priestly portion of dough, hallah, can (outside the Land of Israel) be separated retroactively: after the dough is baked into bread and eaten, what is left over is made hallah, retroactively permitting the non-priest to have eaten the rest of the bread in the first place. (For a non-priest to eat the priest’s portion is a serious violation.) But since, in retrospect, the bread that was baked contained the priestly portion, why do not the standard rules of kashrut dictate that the bread is made unkosher, just as if it had been baked while touching a forbidden substance? On examination, the question turns on subtle issues of identity: are the flavor particles which allegedly contaminate this bread truly one with the hallah retroactively separated? It turns out that this is a dispute between the Taz and his father-in-law, the Bah: the Taz in fact says that in any case of retroactive hallah separation, the rest of the dough must be more than sixty times the volume of hallah separated. See Taz, note 15 to Yoreh De‘ah 325.

The Maharal (Gur Aryeh, Shemos 30:29) also discusses this issue with regard to the mishkan:

ואתה אל תאמר כאשר היה המשכן בבנינו, שכל חשיבתו לא היה רק הקרשים והדברים השייכים לו, שהרי כל אלו דברים הגיע להם שנוי, שכאשר הגיע הפסד ליריעות וקרשים, היום בזה ולמחר באחר, עד שהיה חדש הכל, אל תאמר שהיה זה נקרא משכן חדש, שאין הדבר כך, כי אין חשיבות המשכן הדברים החומרים, רק עצם הציור שהיה מסודר מן השם יתברך, ואל דבר זה לא הגיע בטול ושינוי. ובשביל זה לא נאמר שהיה שינוי למשכן, [ד]כל זמן שעצם הציור קיים אין להקפיד על שינוי הגופות. ולפיכך יש לנו לומר גם כן שיש קיום לכל אלו הדברים שאמרנו, שכמו בעת עשיית המשכן היה מחייב סדר המציאות שיהיו נמצאים, כך חייב עתה הכלים אשר מיוחדים לעלוי מעלתם, ואין הפסד בהם, ואם הגיע הפסד - לא הגיע רק מצד הנושא, כמו שאמרנו. ומפני מעלת ומדריגת השמן, נעשה בשמן המשחה כמה נסים, כמו שמבואר במקומו. ומכל מקום התבאר לך כי הנרות יאירו אל מול פני המנורה לעולם, כי לא הוסר אור עריכתם מן המציאות, ודי בזה למבינים, כי הם דברים ברורים מאד:

  • Very interesting. Please put in the location of the Maharal you quoted.
    – N.T.
    Jun 14, 2021 at 20:15
  • Would you be able to translate the quote form the Maharal Jun 15, 2021 at 0:27

The Gemora Bova Kamma perek Hagozel Eitzem discusses at length when a change (shinui) to an item causes it to lose the status of the original item. There are differences between what is Doraysa and what is Drabonon, manmade versus natural changes etc.

Many have relevance but to quote just one Gemora (Bava Kamma 96B)

אמר רב פפא האי מאן דגזל עפרא מחבריה ועבדיה לבינתא לא קני מאי טעמא דהדר משוי ליה עפרא לבינתא ועבדיה עפרא ןקני מאי אמרת דלמא הדר ועביד ליה לבינתא האי לבינתא אחריתי הוא ופנים חדשות באו לכאן

Rav Pappa said: someone who robbed another of dirt and made it into a brick has not acquired it due to the change. What is the reason for this? It is that he can return it and convert it back into earth. However, if he robbed another of a brick, and by crushing it turned it into earth, he has acquired it due to the change. If you say: Perhaps he will return it and fashion it into a brick? This is a different brick, and a new entity that we have

The Rambam (Hilchos Gezila 3:10) says

שִׁנּוּי הַחוֹזֵר לִבְרִיָּתוֹ אֵינוֹ שִׁנּוּי. כֵּיצַד. הַגּוֹזֵל עֵצִים וְדִבְּקָן בְּמַסְמְרִים וְעָשָׂה מֵהֶן תֵּבָה אֵינוֹ שִׁנּוּי שֶׁהֲרֵי אֶפְשָׁר לְפָרְקָן וְחוֹזְרִין לוּחוֹת כְּשֶׁהָיוּ

A change that can revert to its original state is not considered to be a change. How does this manifest? When a person obtains boards by robbery and attaches them to each other with nails and makes a box, this is not considered a change. For it is possible to separate them and make them simple boards, as they were previously.

Based on the above it would seem that only if someone were to rebuild the exact same ship from the exact same pieces then it would be considered a ship that was dissembled and reassembled and remain the same ship. If however different pieces are being used it is no longer the same ship or ax.

  • 1
    What if you took a boat and replaced a lightbulb. Is it now a different boat? I don't understand how to apply this standard to this problem
    – Double AA
    Jun 14, 2021 at 15:27
  • The ship of Theseus isn't about changing a lightbulb. It is about replacing all or most of the material. Changing a lightbulb isn't a real change. You can easily put back in the old one. If however the boards of a ship were replaced in a manner that can not be undone that would be a change
    – Schmerel
    Jun 14, 2021 at 15:37

Even if you replace all or most of the material, even if we assume that some of the material is completely new, there is still the principle as noted by N.T., that nedarim go by lashon bnei adam.

Say I vow not to ever climb the Eiffel tower, every single piece of which has been replaced at least twice (according to the official site), which technically makes it possible in a lifetime to visit two physically distinct Eiffel towers, I would still be bound by the vow because the original intent is related to the place (Eiffel tower) and not to the actual pieces that compose it.

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