The Mishna teaches that an eiruv tehumin must be extant and edible at the moment that Shabbat begins. If the food gets burned up, or if it is teruma and, before Shabbat begins, it becomes tameh ritually impure) and thus can no longer be eaten, the eiruv is not valid. If the food is destroyed or becomes inedible after Shabbat begins, the eiruv is valid.
If the matter is in doubt, i.e. if he does not know when one of the aforementioned incidents occurred, Rabbi Meir and Rabbi Yehuda say: This person is in the position of both a donkey driver, who must prod the animal from behind, and a camel driver, who must lead the animal from the front, i.e. he is a person who is pulled in two opposite directions. Due to the uncertainty concerning his border, he must act stringently, as though his resting place were both in his town and at the location where he placed the eiruv. He must restrict his movement to those areas that are within two thousand cubits of both locations.
Rabbi Yose and Rabbi Shimon disagree and say: An eiruv whose validity is in doubt is nevertheless valid. Rav Yosei said: The Sage Avtolemos testified in the name of five Elders that an eiruv whose validity is in doubt is valid.
What if we know that the food burned up or became tameh sometime late in the day on Friday, but we do not know whether that occurred before or after Shabbat began? Regarding this situation, the Mishna presents a disagreement. Rabbi Yose rules that a questionable eiruv is valid, and he quotes Avtolemos who supports him in this halakha. Rabbi Meir rules that in this case the individual who sets down the eiruv becomes a Hamar-Gamal – “a donkey-camel driver,” which means that, because of the questionable eiruv, he is limited in both directions. He cannot travel 2,000 amot in the direction that he intended, and he has also lost his ability to travel 2,000 amot outside the city in the other direction.
The expression Hamar-Gamal, which appears a number of times in Massekhet Eiruvin, stems from the different behavior of these two animals and subsequently, the way they are treated by their masters. A donkey is driven from behind and plods along under his burden; a camel, on the other hand, is pulled from the front. Thus someone who is a Hamar-Gamal needs to be in two different places at the same time, and, in effect, cannot move at all. The Aruk describes the term as someone who needs to pull the reluctant donkey and drive the unwilling camel, and therefore will end up rooted to his spot.
Regarding Avtolemos, the sage quoted by Rabbi Yossi, we encounter something of a mystery. He clearly was one of Rabbi Yossi’s teachers, as Rabbi Yossi quotes him not only in our Mishna, but with regard to other areas of halakha, as well. Some identify him with Avtolemos ben Reuven, who was granted special permission by the Sages to dress in the fashion of non-Jews in order to work on behalf of the Jewish community, explaining the unique situation as stemming from his close relationship with the ruling authorities (see Sotah 49b). It is possible that Avtolemos is the son of Reuven ha-Itzrabuli, who played a similar role under the Roman government (see Me’ilah 17a).