[Esau would ask his father:] “How does one tithe straw?”
Midrash Rabbah, Bereishit 63:15
And the house of Jacob shall be fire … and the house of Esau, straw
Straw, being animal fodder, is absolved from the obligation of tithing, which applies only to produce grown for human consumption. Esau inquired of Isaac as to how to tithe straw, wishing to impress his father with a demonstration of copious piety; in doing so, he also demonstrated a fatal flaw in his approach to life.
Man, the Kabbalists tell us, is vitalized by no less than two souls: we each possess both an “animal soul” and a “G-dly soul.” The animal soul is the motor of physical life, incorporating the instincts, drives, desires and faculties that serve the survival and perpetuation of the physical self. In this, the human being is another animal, however intelligent and sensitive an animal. What distinguishes him from the animal world is his G-dly soul, which embodies his striving to transcend the animal state and relate to the Divine.
All endeavors of man are either “human food” that fuels the spiritual life of the G-dly soul, or “animal fodder” that nourishes the material life of the animal soul. Both are indispensable to our purpose in life, for the soul can operate and achieve its goals only via a physical self; but one must take care not to confuse the means with the end. One must always be able to distinguish between the sacred and the mundane in one’s life, and remember which exists to serve which.
Esau wished to tithe the straw of life, to attribute spiritual worth to animal fodder. Instead of exploiting the material to serve the spiritual, he wished to invest the material with a significance and value of its own.
The end, of course, was that the “dew of the heavens and the fat of the land” intended for Esau was given to Jacob, who could be trusted to “tithe” or idealize only what nurtures the man in man—the G-dly drives and aspirations that distinguish the human from the animal.
In the end of days, prophesies the prophet, when the purpose of creation will reach its fulfillment, “the house of Jacob shall be fire… and the house of Esau, straw.”
This is not the same straw of which Esau spoke to Isaac. The Hebrew word used in that connection by the Midrash is teven, while the prophet describes the house of Esau as kash. While both words loosely translate as “straw,” teven is more precisely the chaff that is harvested together with the grain and is subsequently fed to the livestock, while kash is the stubble that remains in the field and is too coarse even for animal consumption.
Esau was initially entrusted with the teven, the straw symbiotically related to the grain—the straw that feeds the animal that serves man. But when he sought to reverse this relationship—to make straw the focus and object of life—his teven turned to kash, into a hollow husk depleted of all nutritive potential.
In the perfect world of Moshiach, such empty materialism will cease to be—the kash of Esau will be utterly consumed by the spiritual fire  of Jacob.Teven, on the other hand, will become the staple of the animal kingdom, as the prophet says, “the lion, like the ox, shall eat straw (teven).”
Based on an entry in the Rebbe’s journal, circa 1942
 Giving a tenth of one’s produce to the Levite or the pauper.
 Rabbi Chaim Vital, Shaar Hakedushah; Etz Chaim, Portal 50, ch. 2; elaborated in Tanya and Chabad Chassidic literature
 Genesis 27:28.
 Talmud, Bava Metzia 103a and Shabbat 36b.
 “My word is as fire, says G-d” in reference to the Torah, the “voice of Jacob” (Jeremiah 23:29; Talmud, Berachot 22a; midrashim on Genesis 27:22).
 Isaiah 11:7.
 Reshimot #19, pp. 7-10.