Ohalot, Chapter Sixteen, Mishnah Four
1) One who searches, must search over a square cubit and then leave a cubit, [digging down] until he reaches rock or virgin soil.
2) [A priest] carrying out earth from a place of uncleanness may eat his terumah mixed with hullin.
3) But one who is clearing away a heap of stones, may not eat his terumah mixed with hullin.
Section one: This section refers to the person in yesterday’s mishnah who had found three corpses buried in the ground and was searching to determine how far the burial ground extended. He need not search every cubit of earth. Rather, he can check a cubit and then leave a cubit, because it was customary to leave a cubit between each body buried in the ground.
When checking, he must dig down in the ground until he either reaches rock or soil that clearly has never been dug up before.
Section two: This section concerns a priest who is removing earth from a place that has some impurity in it, such as the burial ground referred to in mishnah three. If this person is a priest he may eat terumah that has become mixed in with hullin (non-sacred produce). This is because the earth that he is carrying is not certainly impure, and the terumah that he is eating is also not certainly terumah. Since there is a “double doubt” he may eat the terumah.
Section three: However, if one is clearing away a heap of stones that fell on a person and might have killed him, he may not eat such terumah. The problem is that if he finds a corpse, he will certainly be impure. In other words, although there is some doubt in this case as well, it is of a different nature from the doubt in the previous case. In the previous case, the terumah mixed in with hullin will always remain of doubtful status. There is simply no way to determine whether it is terumah or hullin because it is all mixed up. However, in this case, the doubt can be clarified. If he sees that there is a corpse under the stones then the priest is definitely impure. Therefore, he cannot eat the terumah.