I noticed that sometimes these two are used interchangeably, especially as designations of older, Medieval written documents. I know that 'epistle' is a letter, whereas 'treatise' is a written work concerned with a specific subject; surely, they are not synonymous. But can they ever be mutually inclusive? Any information regarding these two words is appreciable.

  • A treatise can be published as a book or can be addressed - in a letter - to an individual or a company of individuals. In which case it will also be an epistle. Paul's epistle to the Roman church is a treatise. His epistle to Philemon is not, but is personal.
    – Nigel J
    Apr 17, 2018 at 15:34

1 Answer 1


In layman's terms an epistle is a 'letter'; whereas a treatise is essentially an 'argument in writing'. (https://www.merriam-webster.com/dictionary/epistle) (https://www.merriam-webster.com/dictionary/treatise)

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