Commentary for Avodah Zarah 139:8

The question the Talmud asks is how come the non-Jew does not know that the Jew will keep Shabbat. The answer, quite fascinatingly, is that indeed, non-Jews do not realize that Jews keep Shabbat. A convert named Issur says that before he converted, he and his fellow non-Jews assumed that Jews do not keep Shabbat, for if they did, then purses would be found in the marketplace all the time, for Jews could not carry them! What he did not know is that R. Yitzchak taught a way that a Jew could indeed pick up a purse and carry it four cubits at a time and thus save it from the marketplace. The purse referred to here is probably not just a random one, but rather a purse that the Jew was carrying when Shabbat began. If there is no alternative but to just toss it, the Jew can carry it four cubits at a time to get it home. But this is only allowed for one’s own purse/wallet.