There are four [cases of] doubt in which R' Yehoshua rules [that it is] tamei and the Sages rule [that it remains] tahor. How so? (a) A tamei person is standing and a tahor person passes by; (b) a tahor person stands and a tamei person passes by; (c) [an object of] tumah is in the private domain and [an object that is] tahor is in the public domain; (d) [an object that is] tahor is in the private domain and [an object of] tumah is in the public domain. [These are the four cases.] [There is] a doubt whether [a person] touched [it] or did not touch [it], [or] there is a doubt whether [one entity] formed a roof [over them] or did not form a roof [over them], [or there is] a doubt whether [he] moved [the tamei] or did not move it. [In each of these four cases,] R' Yehoshua rules that [the person] becomes tamei, but the Sages rule [in each situation that he] remains tahor.