The principle of unity is fundamental to Judaism, expressing itself in each of the major concepts of our faith: G‑d, Torah, and Israel.

Achdut Hashem, the Unity of G‑d, is a fundamental and all- comprehensive principle of Judaism, second only to existence of G‑d (and in a way including that as well). We affirm it twice daily with the mitzvah of reciting the Shema, "Hear, O Israel, G‑d, our G‑d, G‑d is One!" It implies a unique and pure oneness in an absolute sense, with nothing at all comparable to it. To deny this oneness is to deny the very essence of our faith, all the precepts of the Torah, for they all depend on belief in G‑d and His unity. Quite obviously this principle embraces also the prohibition of idolatry, the prohibition of assuming that there is any other power or reality besides G‑d.

Idolatry is the very opposite of unity. It sets up a multiplicity of things, pluralism, realities outside and separate from G‑d. Hence it is the worst sin: to believe in multiplicity is to acknowledge idolatry - which in turn is to deny the whole Torah.

As G‑d is the very principle of absolute unity, His creation and effects, too, manifest unity: from One can only come one.

Maimonides thus devotes a lengthy, and widely celebrated, chapter in Moreh Nevuchim- (Guide to the Perplexed), to show how the universe demonstrates in its entirety a unity like that of an individual being. Whatever differences there are between its substances are but like the differences between the limbs of a person. Just as a person is one individual, and at the same time composed of the various parts of the body (such as the flesh, bones, the various mixtures etc.), so is the universe.

"This representation of the whole of the sphere as one living individual and possessing a soul....is most necessary or useful for the demonstration that the Deity is One .... and will also clarify the principle that He who is One has created a unitary being."

Even as the principle of yichud (unity of G‑d) underlies the religious life of Halachah, and avodah zara (idolatry) is its very antithesis, so, too, in the philosophical system of Jewish mysticism.