Pesachim 46A

לענין צירוף טומאה בפסח, ובשאר ימות השנה איכא פלוגתא.

with regard to the combination of two pieces vis-à-vis ritual impurity during Passover, when it depends upon their volume. However, during the rest of the year there is a distinction based upon whether the owner is particular about it or not.

היכי דמי, כגון דאיכא פחות מכביצה אוכלין, ונגעו בהאי בצק. בפסח, דאיסורו חשוב ⁠— מצטרף. בשאר ימות השנה, דבקפידא תליא מילתא, אם מקפיד עליו ⁠— מצטרף, אם רוצה בקיומו ⁠— הרי הוא כעריבה.

The Talmud explains: What are the circumstances of the Mishnah’s case? It is a case where there is less than an egg-bulk of ritually impure food, and it touched this dough in the bowl, and then it came into contact with ritually pure food. During Passover, when the prohibition that applies to the dough causes it to be considered significant although it is a very small quantity, it combines with the first piece of food. Together they are the size of an egg-bulk, which is able to transmit the ritual impurity of foods. However, during the rest of the year, when there is no prohibition that imparts this significance to the dough, the matter is dependent on the owner’s particularity; if he is particular about it, i.e., he does not want the dough to be there, it is considered food rather than part of the bowl, and it combines with the other piece of food. However, if one prefers its continued presence in its current location, it is considered like part of the kneading bowl itself, rather than food.

מתקיף לה רבא: מי קתני ״מצטרף״? והא ״חוצץ״ קתני. אלא אמר רבא: וכן להעלות טהרה לעריבה.

Rava strongly objects to this: Was the language taught in the Mishnah: Combines? Didn’t the Mishnah teach that it interposes? Abaye’s explanation does not account for this term. Rather, Rava said that the Mishnah should be understood as saying: And so too with regard to purifying the kneading bowl via immersion.