"…The man [Isaac] became great and kept becoming greater until he was very great." (Gen. 26:13)
For they would say, "The manure of the mules of Isaac, and not the silver and gold of Abimelech."
(Rashi on verse, based on Bereishit Raba 64:7)

The point of the Midrash that Rashi quotes is that when people used to want to describe excessive wealth they would describe it in terms of the silver and gold possessed by Abimelech. (Bereishit Rabbah 64:7) After Isaac had become rich, they described excessive wealth in terms of the dung of Isaac's mules. This seems a curious comparison. How can we assume that Abimelech's wealth did not even amount to the value of the dung of Isaac's mules? Another difficulty is the Midrash's comment on the words "found the 'yeymin'". (Gen. 36:24) These yeymim are described as half-donkey and half-horse. (Bereishit Rabba 82:15) Anah who crossbred horse and donkey to produce mules is severely criticized for interfering with G‑d's plan of maintaining the purity of the species. How can we assume that Isaac 1) kept or raised such animals and 2) was the first one to crossbreed, seeing that the Torah reports this as something new at a later period, during Jacob's lifetime? Isaac's extraordinary success…served to show that he was indeed a true son of Abraham

The scoffers of that generation maintained that Sarah had been impregnated by Abimelech and that Isaac was the issue. They claimed that, had Abraham been Isaac's father, it would have been impossible for Isaac to have fathered a son such as Esau who pursued married women to commit adultery with them under the noses of their husbands. Isaac's extraordinary success, the fact that he was sanctified on Mount Moriah, and his life-long residence on the holy soil of the Land of Israel, all served to show that he was indeed a true son of Abraham.

Of Esau, the Torah reports, "Esau went to a land". (Gen. 36:6) He had to leave the Land of Israel because that land "vomits" people who behave in a grossly incestuous manner. Isaac was the model of refinement in every respect. The reason G‑d blessed him in such an extraordinary fashion was to demonstrate the fact to one and all that sexually pure conduct unlocks all the bounty of the land of Israel to those who dwell in it. Isaac possessed mules which were not the result of crossbreeding…

How did G‑d prove that point? Whereas in the days of Anah a mule had to be the product of crossbreeding, Isaac became the owner of mules which were not the result of crossbreeding but a heavenly gift. Even the scoffers who lived around him became aware of this. This is why Isaac became "great" in their eyes. They realized that if G‑d performs such miracles with the livestock of a tzadik, how much greater would be the miracles He would perform with the tzadik himself! When the people said that the dung of Isaac's mules was worth more than the silver and gold of Abimelech, they meant that the fact that Isaac possessed mules which were not the result of crossbreeding proved more about his status as a holy man than the thousand silver pieces Abimelech had given to Sarah to demonstrate that her virtue had not been compromised while she was in Abimelech's palace. (Gen. 20:16)

[Translated and adapted by Eliyahu Munk.]