late 14c., "unaccented syllable or note," from Latin thesis "unaccented syllable in poetry," later (and more correctly) "stressed part of a metrical foot," from Greek thesis "a proposition," also "downbeat" (in music), originally "a setting down, a placing, an arranging; position, situation," from root of tithenai "to place, put, set," from PIE root *dhe- "to set, to put" (see factitious). Sense in logic of "a formulation in advance of a proposition to be proved" is first recorded 1570s; that of "dissertation presented by a candidate for a university degree" is from 1650s.