The Encyclopedia of the Brethren of Purity ( Rasa'il ikhwan as-safa' wa khillan al-wafa; also variously known as the "Epistles of the Brethren of Sincerity", "Epistles of the Brethren of Purity" and "Epistles of the Brethren of Purity and Loyal Friends") was a large encyclopedia

Encyclopedia

An encyclopedia is a type of reference work, a compendium holding a summary of information from either all branches of knowledge or a particular branch of knowledge....

 in 52 treatise

Treatise

A treatise is a formal and systematic written discourse on some subject, generally longer and treating it in greater depth than an essay, and more concerned with investigating or exposing the principles of the subject.-Noteworthy treatises:...

s (or "rasa'il") written by the mysterious Brethren of Purity

Brethren of Purity

The Brethren of Purity were a secret society of Muslim philosophers in Basra, Iraq, in the 10th century CE....

 of Basra, Iraq sometime in the second half of the 10th century CE

Common Era

Common Era ,abbreviated as CE, is an alternative designation for the calendar era originally introduced by Dionysius Exiguus in the 6th century, traditionally identified with Anno Domini .Dates before the year 1 CE are indicated by the usage of BCE, short for Before the Common Era Common Era...

 (or possibly later, in the 11th century).