a letter written for a didactic purpose
An epistle (; Greek: ἐπιστολή, epistolē, "letter") is a writing directed or sent to a person or group of people, usually an elegant and formal didactic letter. The epistle genre of letter-writing was common in ancient Egypt as part of the scribal-school writing curriculum. The letters in the New Testament from Apostles to Christians are usually referred to as epistles. Those traditionally attributed to Paul are known as Pauline epistles and the others as catholic (i.e., "general") epistles.
The epistles of Christian literature have their parallels in Islamic literature as well, in which the word risāla (plural, rasāʾil) is commonly translated as epistle. The Epistles of the Brethren of Purity (Rasāʾil Ikhwān al-Safa) was a collection of fifty-two epistles written in the tenth century spanning the topics of psychology, philosophy, mathematics, metaphysics and moral fables. The Epistle of the Right Path (Risāla-yi Sirat al-Mustaqiım) is the oldest Persian Ismaili prose work found in the post-Mongol era. It, in turn, references the Epistle of Sorrow, (Risālat al-Huzn), written by the Ismaili Imam ʿAbd al-Salām.
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