Last week's parsha introduced the issur of a kohen becoming tamei meis, except to bury certain close relatives. The Marcheshes (long footnote at the end of siman #2) raises the following chakirah: can a kohen enter a reshus ha'rabim where there is a safeik tumas meis? Does the din of safeik tumah b'reshus harabim tahor mean that there is no tumah, and therefore the kohein can l'chatchila enter the area, or does safeik tumah b'reshus ha'rabim mean that even though there may be potential tumah, ex post facto, we treat someone who entered such a place as tahor -- only b'dieved?
The Marcheshes was not a Brisker (see his hakdamah!), but the chakirah echoes a gavra/cheftza type split. Is safeik tumah b'reshus harabim a din in the tumah, the cheftza, not really being tumah, or is it just a din in the gavra remaining tahor even though he might have in fact encountered something tamei?
The Rambam writes in Hil Aveil 3:6
כהן גדול אינו מטמא לקרובים שנאמר לאביו ולאמו לא יטמא. וכן אינו נכנס עם המת באהל אפילו קרוביו. שנאמר ועל כל נפשות מת לא יבא. הא למדת שהוא חייב בלא יבא וחייב בלא יטמא
There are two different issurim involved here: 1) לא יטמא and 2) ועל כל נפשות מת לא יבא
The Marcheshes suggests that viz a viz the issue of לא יטמא, all that we look at is the end result -- is the kohen causing himself, the gavra, to become halachically tamei or not. Since safeik tumah b'reshus ha'rabim does not cause a state of tumah, there is no issur.
However, when it comes to the issur of ועל כל נפשות מת לא יבא, here we are not speaking about the end result, but rather about physical contact and proximity to a meis. Since there is a cheftza of safeik tumah present, the kohen must l'chatchila avoid the area.
The flipside applies to a kohen who entered a reshus ha'yachid where there is safeik tumah. Since the halacha says the gavra is tamei, לא יטמא has been violated. However, since the cheftza is safeik tumah, there would be no malkos for the lav of ועל כל נפשות מת לא יבא.
The Marcheshes then adds some icing onto the cake. He writes that he heard b'shem R' Chaim that even though safeik tumah b'reshus ha'yachid is treated as a vaday, that is not the same as a vaday of tumas **meis**. There is a difference between actual tumas meis and a din tumah. If a kohen comes in contact with an actual body, then the lavim of לא יטמא and ועל כל נפשות מת לא יבא kick in. However, if the kohen comes to a safeik tumah b'reshus ha'rabim, **legally** it is as if the kohen became tamei, but that legal reality is not the same as actually coming into contact with a body. Therefore there would be malkos for neither lav.