Shabbat 112

Ship of Theseus.

The ship of Theseus, a well-known ancient philosophical thought experiment, asks: if a ship undergoes continuous repairs such that each piece of the ship is replaced one at a time, is it in the end the same ship? Today’s daf offers an answer.

The Gemara on today’s page cites a mishnah from tractate Kelim which teaches that when a wooden object becomes impure, it can become pure again if it acquires a hole the size of a pomegranate. The hole renders the object different enough that it is now a new object, and therefore pure. Rabbi Hizkiya explores this idea further:

Hizkiya asked: What is the halachah when a utensil was perforated with a hole large enough for an olive to emerge, and he sealed it, and then it was perforated again with a hole large enough for an olive to emerge, and he sealed it, and this went on until the total area of all the holes completed a space large enough for a pomegranate to emerge?

Our imagined object does not have one hole the size of a pomegranate, but many olive-sized holes that add up to the size of a pomegranate. And, what’s more, they are made and sealed serially, so that not all holes exist simultaneously. Is this enough of a transformation to render the object once again pure?

Rabbi Yohanan, a student and colleague of Rabbi Hizkiya, offers the answer:

Master, you taught us that with regard to a sandal that became ritually impure and one of its straps broke and he repaired it, this sandal remains ritually impure with impurity imparted by treading. If the second strap broke and he repaired it, the sandal is ritually pure . . . and you said to us that the reason the sandal is pure is because it has a “new face.” Here too, a new face has arrived.

In the case of a shoe whose straps break and are replaced one at a time, the shoe loses its original status of impurity because it is a new entity. Similarly, reasons Rabbi Yohanan, in the case of the wooden object, the creation of small holes that are then repaired render it a new entity and it thereby loses its original status of impurity.

The original topic on today’s page was tying knots on Shabbat. Why does the Talmud bother with this tangent? It may be here simply because the mishnah in tractate Kelim was mentioned earlier on our daf and the Gemara brought another teaching about it (incidentally, there is no Gemara to Mishnah tractate Kelim). But I believe there is also a conceptual reason for this digression.

The ship of Theseus raises questions of the permanence and transience of objects. Can an object appear relatively static while its nature drastically changes? Rabbis Hizkiya and Yohanan answer this question with a resounding “yes” and thus help inform our discussion of knots as well. As we strive to construct a clear principle defining permitted and forbidden knots on Shabbat, this thought experiment suggests we look at the nature of each knot. Is it static, unchanged over time, or is it transient, a knot which is tied over and over again? While the knot may appear the same, it in fact becomes a new entity each time it is tied and untied.

This is a classic Talmudic maneuver. On the surface, the conversation may seem tangential and irrelevant, but with time, the tangents reveal themselves to offer new insight to the central topic, providing new language and concepts to inform future discussions.

Read all of Shabbat 112 on Sefaria.

This piece originally appeared in a My Jewish Learning Daf Yomi email newsletter sent on June 26, 2020. If you are interested in receiving the newsletter, sign up here.

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