In the context of teaching that an animal that had become sanctified by means of temura (i.e. the owner of a sanctified animal attempted to switch the sanctity onto another animal so that ultimately each become sanctified) cannot cause another animal to become sanctified by means of temura – ein temura oseh temura – the Mishna on today’s daf lists a number of similar laws. One of them is the rule that ein beit haperas osin beit haperas – one beit haperas does not create another beit haperas.
A beit haperas is a place the Sages decreed ritually impure, as though it were a cemetery. The law of a beit haperas teaches that a field that has doubtful status regarding ritual defilement of a dead body must be treated stringently. When we know that there was a grave in a given field, when that field is plowed the doubtful ritual status may be created either because we fear that the plowing may have dislodged bones that now may be spread throughout the field or that the grave was “lost” and we cannot be certain where the body is buried.
With regard to the source of this expression, Rashi and Tosafot agree that the word peras means “broken” and that we fear that the original grave and its contents have been broken by the plowing. According to the Rambam, the term peras means “spread out,” since we treat the field as if the ritual defilement is spread across the entire field. Others suggest that the source may be Greek expressions meaning “courtyard” or a place that cannot be crossed.
The rule that ein beit haperas osin beit haperas teaches that if someone has adjoining fields, one of which contains a grave, if that field is plowed giving it the status of a beit haperas, plowing from there into the adjoining field will not cause that field to become a beit haperas, as well.