פירוש על טהרות 4:7

ספק מים שאובים – it is doubtful if the drawn waters fell into this Mikveh/ritual bath and invalidated it or they didn’t fall, or that we saw that drawn waters fell into it, that there is a doubt if there were three LOG which is the measure to invalidate the Mikveh, or that there weren’t in them according to this measure. And all of the rest of the doubts are explained further in [the Mishnayot] in our chapter.

These are the cases of doubtful uncleanness that the sages declared to be clean:
A doubt concerning drawn water for a mikveh,
A doubt concerning an object of uncleanness that floated upon the water.
A doubt concerning liquids as to whether they have contracted uncleanness it is deemed unclean, but if it was whether uncleanness has been conveyed it is deemed clean.
A doubt concerning the hands as to whether they have contracted uncleanness, have conveyed uncleanness or have attained cleanness, they are deemed clean.
A doubt that arose in a public domain;
A doubt concerning an ordinance of the scribes;
A doubt concerning non-sacred food;
A doubt concerning a sheretz;
A doubt concerning negaim;
A doubt concerning a nazirite vow;
A doubt concerning a first-born;
A doubt concerning sacrifices.
This mishnah introduces all sorts of cases of doubt where the sages ruled that something was clean. Except for the first case, the remainder of the list is explained from mishnah eight until the end of the chapter. Therefore, we will not explain these sections here.
Section one: If drawn water falls into a mikveh, the mikveh is disqualified. If there is a doubt whether this occurred, the mikveh remains valid.
Sections 2-12: These are explained in the following mishnayot. Stay tuned!