Ketubot 24: Collusion or ritual matters? and How does he prove that he's a Kohen?

Carrying on from yesterday's discussion, today's daf begins with commentary on why different case examples are provided.  The rabbis note a concern regarding women colluding and providing misleading testimony.  They also consider how the case of men of unknown lineage who claim that they are priests has different implications for purposes of finances and of ritual.  Rabbi Yehuda and the rabbis disagree about this.  The Gemara attempts to deconstruct the difference between their arguments.

The Gemara compares this case to that of two men bringing produce to a town on their donkeys.  If the first to arrive praises his colleague's produce and diminishes his own (ex. my produce is fresh and still moist while his is dry; my produce is not tithed while his is demai, doubtfully tithed), he is not believed.  The rabbis are said to be in collusion with each other; at the next town they will switch roles.  While there is suspicion of collusion, the rabbis rule leniently in these cases.

Another case is introduced regarding ritual purity.  If a potter has left his vessels on their own and he leaves for a drink of water, those pots might be touched by passersby, for the pots are close to the public domain.  

The Gemara wonders what Rabbi Yehuda and the rabbis are disagreeing about.  Is it about collusion?  Or is it about whether one who eats teruma is permitted to be 'elevated' to the full status of priest, ie. permitted to marry a kohenet, etc.  The Gemara goes on to ask a number of questions about the limitations on one who calls himself a priest.  Can a person who signs a document as a priest allowed to use that as proof of his priestly lineage?  Can a person who raises his hands in the priestly benediction  automatically understood to have priestly lineage?  No, we learn, these priestly actions do not prove priestly lineage.

Our daf ends with a number of possible explanations as to why the lineages of these priests are questioned to such a degree.  The rabbi cite a number of different concerns.  Primary is the notion of presumptive status.  Rabbi Yosei reminds us that the authority of presumptive status is great.  These men were of unknown lineage, and that outweighs their behaviour - including performing the priestly benediction.