Shayast ne-Shayast.'proper and improper'.one of the Middle Persian

Middle Persian

Middle Persian , indigenously known as "Pârsig" sometimes referred to as Pahlavi or Pehlevi, is the Middle Iranian language/ethnolect of Southwestern Iran that during Sassanid times became a prestige dialect and so came to be spoken in other regions as well. Middle Persian is classified as a...

 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

Mysticism

Mysticism is the knowledge of, and especially the personal experience of, states of consciousness, i.e. levels of being, beyond normal human perception, including experience and even communion with a supreme being.-Classical origins:...

 signification of the Gathas

Gathas

The Gathas are 17 hymns believed to have been composed by Zarathusthra himself. They are the most sacred texts of the Zoroastrian faith.-Structure and organization:...

, and praise of the sacred beings are the principal subjects discussed.