Here this is expressed in a letter by the Lubavitch Rebbe...
http://www.sichosinenglish.org/cgi-bin/calendar?holiday=purim1415By the Grace of G-d
28th of Adar, 5721
Brooklyn, N.Y.
Greeting and Blessing:
....The unity of the material and spiritual, to which I referred above, is also one of the features of Purim. For Haman's decree began with an attack on the spiritual freedom of the Jews, as our Sages explain the verse "But Mordechai did not bend his knee nor bow down to Haman," who wished to impose his idolatry upon all, and indeed succeeded, except for Mordechai. But then his decree extended to the physical annihilation of all the Jews, young and old, children and women.
That is why the miracle of Purim is observed both spiritually and materially, with light, and gladness, and joy and glory, which our Sages explained in a spiritual sense - Light that is the Torah, etc., and at the same with a Seudah, with wine, etc.
Indeed, the principle of unity is the essence of Judaism, since Abraham first proclaimed Monotheism in a world of idolatry, which came to full fruition at the Revelation at Mount Sinai. For true Monotheism as professed by us and as explained in the Jewish religion is not only the truth that there is only One G-d and none with Him, but that there nothing besides (Ein Od), that is the denial of the existence of any reality but G-d's, the denial of pluralism and dualism even the separation between the material and spiritual.
It is interesting to note that the more the physical sciences advance, the closer one approaches the principle of unity even in the world of matter. For, whereas formerly it was the accepted opinion that the plurality and compositeness in the material world can be reduced to some 100 odd basic elements and entities, and physical forces and laws were regarded as being separate and independent, not to mention the dichotomy between matter and energy. But in recent years, with the advancement of science, the basic elements themselves were reduced to several more elementary components of the atoms, viz. electrons, protons and neutrons, and even these were immediately qualified as not the ultimate blocks of matter, until the discovery was made that matter and energy are reducible and convertible one into the other.
It is well known that the Baal Shem Tov, the founder of Chassidus, taught, and the Alter Rebbe, the founder of Chabad, explained and amplified, that every detail in human experience is an instruction in man's service to His Maker. Thus, what has been said above about the advancement of science, exemplifies also the progress of human advancement in the service of G-d.
Man possesses two apparently contradictory elements, no less incompatible than the incompatibility of matter and spirit, the counterpart of which in the physical world is matter and energy. I refer to the Divine soul and animal soul, or, on a lower level, the Yetzer Tov and Yetzer Horah. But this incompatibility is evident only in the infantile stage of progress in Divine service, comparable to the plurality of elements and forces which were presumed to exist in physical Nature. But just as the appreciation of the underlying unity of Nature grew with the advancement of science, so does perfection in the Divine service lead to the realization of the essential unity in human nature, to the point where the Yetzer Tov and Yetzer Horah become one, through the transformation of the Yetzer Horah by and into the Yetzer Tov, for otherwise, of course, there can be no unity and harmony, since all that is holy and positive and creative could never make peace and be subservient to the unholy, negative and destructive. And in this attained unity the Jew proclaims, Hear, O Israel, G-d our G-d, G-d is one.
This is also what our Sages meant, when they succinctly said - as they often compress far-reaching ideas in a few concise words - that the words "And thou shalt love G-d, thy G-d, with all thy heart" which immediately follow Shema Yisroel, mean: with both your Yetzorim, with the Yetzer Horah, as with the Yetzer Tov.
With blessing
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