Toharot, Chapter Four, Mishnah Eleven
1) “If there is doubt concerning the hands as to whether they have contracted uncleanness, have conveyed uncleanness or have attained cleanness, they are deemed clean.”
2) “Any doubt that arose in a public domain is deemed clean.
3) “A condition of doubt concerning an ordinance of the scribes”:
a) [For instance, he is uncertain whether] he ate unclean food or drank unclean liquids, whether he immersed his head and the greater part of his body in drawn water, or whether there fell on his head and the greater part of his body three log of drawn water, such a condition of doubt is deemed clean.
b) But if a condition of doubt arose concerning a father of uncleanness even though it was only rabbinical, it is deemed unclean.
Section one: This section is just a quote of mishnah seven without any expansion. It is explained in Yadayim 2:4 (a whole tractate about the impurity of hands!). For now it is enough to note that any case of doubt involving the impurity of hands is ruled leniently.
Section two: This clause will be explicated in chapters five and six.
Section three: If there is a doubt and the impurity is only of rabbinic ordinance (derabanan) then the doubt is ruled clean. The mishnah gives a couple of examples of this. The first is when a person ate or drank something and is not sure if it is unclean. A person is only impure “derabanan” if he eats or drinks something impure. The second is concerning drawn water. The rabbis decreed that drawn water defiles if a person immerses his head and most of his body in them or if three log of drawn water falls on him (we shall learn more about this in Tractate Mikvaot). In both of these cases, the doubt is deemed pure.
But if the doubt involved a “father of uncleanness” whose provenance is derabanan, then the doubt is unclean. For example, “mixed blood” which is blood that came out of a person, some when he was alive (doesn’t defile) and some after his death (does defile). Such blood is a “derabanan father of uncleanness.” In this case, although the doubt is only derabanan, it defiles.