I was learning Pesachim, and on 17b I saw Tosafos say the following:

דבתוך הכלי ודאי אסור לטבול מדאורייתא

It is definitely biblically forbidden to immerse something within a vessel (as opposed to a mikveh)

I was very surprised by this phraseology. I would have said instead that immersing something in a vessel filled with water doesn't work, not that it is forbidden. Why does Tosafos say it is forbidden? Is this just an interesting way of saying it doesn't work? Or it's referring to the fact that it will lead to forbidden things, such as someone spiritually impure eating kodshim or entering the Temple? Or is there a bona fide prohibition. If so, what is it?

I did a search on Otzar HaChochmah using the wording of Tosafos and found many Rishonim use this expression. This enlarges my question.

  • Worth noting that the parallel Tosafos in Bava Basra 66b does not use that expression. Maybe it means if someone immersed in a vessel leaves everything with its original prohibitions caused by the Tumah.
    – N.T.
    Feb 17, 2022 at 7:37
  • Perhaps, and this might be a stretch, the prohibition is to make more tumah since the water will become tomei if the kli is an av hatumah.
    – Chatzkel
    Jun 17, 2022 at 15:23

1 Answer 1


Not sure but.... Gemorah in chaggiga (:כב) explains that toveling inside a Keli without the right size opening won't work and that were worried that the am-hoorez will lend his tomeh Keli to a chaver. This may cause large problems with kodshim as you suggested

  • Thank you for the suggestion. Chagigah is discussing immersing a vessel inside a vessel inside a mikveh. This Tosafos is referring to immersing something into a vessel full of water, ie: it's not a mikveh. Further, Chagigah is a derabanan but Tosafos is discussing a doraisa. I'll clarify the question.
    – robev
    May 28, 2020 at 3:56

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