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Shāyast ne-Shāyast (Persian: شایست نشایست, "Proper and Improper") is one of the Middle Persian books which is written about 8th century B/C and containing about 13,700 words. this book is a compilation of miscellaneous laws and customs regarding sin and impurity, with other memoranda about ceremonies and religious subjects in general. The content of this collection are of a very varied character, but sins and good works, precautions to avoid impurities, details of ceremonies and customs, the mystic signification of the Gathas, and praise of the sacred beings are the principal subjects discussed.