Toharot, Chapter Six, Mishnah Two
1) There are four cases of doubt which Rabbi Joshua ruled are unclean and the sages rule are clean.
2) How so? If an unclean man stood and a clean man passed by or the clean man stood and the unclean one passed by; or if an unclean object was in a private domain and a clean one in the public domain or the clean object was in the private domain and the unclean one in the public domain, and there is doubt whether there was contact or not, or whether there was overshadowing or not, or whether there was shifting or not:
a) Rabbi Joshua rules that the clean becomes unclean,
b) But the sages rule that the clean remains clean.
There are four cases here:
1) An unclean man is standing and a clean man passes by him and he may have been defiled by contact, overshadowing or shifting.
2) The same situation but the clean man is standing and the unclean one is moving.
3) There is an unclean object in the private domain and an clean one in the public domain and the clean one may have had contact with the unclean one.
4) The same situation but the positions are reversed.
Rabbi Joshua says that since at least one of the elements was in the private domain, the doubt is impure.
The other sages say that for impurity to occur both elements have to be in the same situation within the private domainboth must be moving or both must be standing. Since in all of these cases one element is either not in the private domain or one element is moving and the other standing, there is no impurity.