In the previous post we discussed a case where there is a safeik whether a baheres preceded the white hair growing from it (in which case the nega is tamei) or the white hair came first and then the baheres grew (in which case the nega is tahor). R' Yehoshua paskened tahor based on chezkas haguf, the assumption of status quo that a person is physically nega-free until proven otherwise.
Achronim ask why this is called a chezkas haguf. A chezkas haguf relates to physical status quo, as opposed to legal status quo, e.g. chezkas taharah. In this case the physical status quo has obviously changed -- both white hair and a baheres which had not previously been on the person's body are now present. The question is not whether the physical body has changed, but rather when and in what order that change occurred. You have a long Shabbos to think this one over... : )