Chullin 10 - May 10, 23 Iyar

If it is discovered after shechita that the knife has nicks, can we assume the slaughter was valid because the nicks occurred after the two simanim were cut, or is there a concern that they occurred while cutting the hide before the simanim? Rav Huna and Rav Chisda disagree on this matter. Two difficulties are raised against Rav Chisda's ruling to permit, but they are resolved. From where do we derive the principle that an item retains its status until proven otherwise (chazaka)? Rabbi Yonatan derives it from the case of a leprous house, but Rav Acha disagrees with his derivation.