Reference for Chullin 258:2

אמר ליה רבי זירא לרבי אסי

R'Zera said to R'Assi, perhaps the reason for R'Simeon's view there [in the first clause] is: since it is attached it is regarded as one with it.<span class="x" onmousemove="('comment',' Lit., 'since it is attached, it is attached'. I.e., as long as it is joined to the living animal, however slender the attachment may be, it is still regarded as part of the living animal and as such cannot be unclean.');">²</span>

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