What is Sefer Yetzirah, The Book of Creation?

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Sefer Yetzirah (Hebrew, "Book of Formation," or "Book of Creation," ספר יצירה) is the title of the earliest extant book on Jewish esotericism. "Yetzirah" is more literally translated as "Formation"; the word "Briah" is used for "Creation"

The "Sepher Yetzirah," or "Book of Formation," is perhaps the oldest Rabbinical treatise of Kabalistic philosophy which is still extant. The great interest which has been evinced of 1ate years in the Hebrew Kabalah, and the modes of thought and doctrine allied to it, has induced me to translate this tractate from the original Hebrew texts, and to collate with them the Latin versions of mediaeval authorities; and I have also published An Introduction to the Kabalah which may be found useful to students.

Three important books of the "Zohar," or "Book of Splendour," which is a great storehouse of Kabalistic teaching, have been translated into English by S. L. MacGregor Mathers, and the "Sepher Yetzirah" in an English translation is almost a necessary companion to these abstruse disquisitions: the two books indeed mutually explain each other.

The "Sepher Yetzirah," although this name means "The Book of Formation," is not in any sense a narrative of Creation, or a substitute Genesis, but is an ancient and instructive philosophical treatise upon one aspect of the origin of the universe and mankind; an aspect at once archaic and essentially Hebrew. The grouping of the processes of origin into an arrangement, at once alphabetic and numeral, is one only to be found in Semitic authors

Part of the Kabbalah - a gnostic Jewish writing.