Treatise
Treatise , noun
[Old English tretis, Old French treitis, traitis, well made. See Treat.]
1.
A written composition on a particular subject, in which its principles are discussed or explained; a tract. — Chaucer
He published a treatise in which he maintained that a marriage between a member of the Church of England and a dissenter was a nullity.
A treatise implies more form and method than an essay, but may fall short of the fullness and completeness of a systematic exposition.
2.
Story; discourse. [Rare] — Shakespeare