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.


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 '20 at 3:56

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