Kelim, Chapter Nine, Mishnah Three
1) If a sheretz was found beneath the bottom of an oven, the oven remains clean, for I can assume that it fell there while it was still alive and that it died only now.
2) If a needle or a ring was found beneath the bottom of an oven, the oven remains clean, for I can assume that they were there before the oven arrived.
3) If it was found in the wood ashes, the oven is unclean since one has no ground on which to base an assumption of cleanness.
Section one: Underneath the bottom of the oven does not count as the air-space of the oven. Therefore, the dead sheretz found there does not defile the oven. We can assume that the sheretz was still alive while it passed through the air-space of the oven, and did not defile the oven on its way down. Thus the oven is clean.
Section two: Similarly, if an unclean needle or ring is found underneath the oven, we need not assume that it made its way there through the oven’s air-space, defiling the oven while it was going down.
Section three: However, if the needle or ring is found in the midst of the oven’s ashes, even if it is underneath the oven, it defiles the oven, because we cannot assume that it was there before the oven was put there. Had it been there before the oven, it should have been underneath the ashes, not in them.