Tractate Tahorot - Chapter 6 - Mishnah 4 טהרות פרק ו׳ משנה ד׳

However many doubts and doubts about doubts that you can multiply, a condition of doubt in a private domain is unclean, and in a public domain it is deemed clean. How so? If a man entered an alley and an unclean object was in the courtyard, and a doubt arose as to whether he entered or did not enter [the courtyard]; Or if an object of uncleanness was in a house and there is doubt whether he entered or not; Or even if he entered, there is doubt whether the uncleanness was there or not; Or even if it was there, there is doubt whether it consisted of the prescribed minimum or not; Or even if it consisted of the prescribed minimum, there is doubt whether it was unclean or clean; Or even if it was unclean, there is doubt whether he touched it or not; Any such condition of doubt is deemed unclean. Rabbi Eliezer says: if there is a doubt whether he entered, he is clean, but if there is a doubt whether he touched it, he is unclean.