On the meaning of ἄρσις "In music, arsis is an unaccented note (upbeat), while the thesis is the downbeat.[3] However, in discussions of Latin and modern poetry the word arsis is generally used to mean the stressed syllable of the foot, that is, the ictus"
The ancient Greek writers who mention the terms arsis and thesis are mostly from rather a late period (2nd-4th century AD), but it is thought that they continued an earlier tradition. For example, it is believed that Aristides Quintilianus (3rd or 4th century AD) adopted much of his theory from Aristotle's pupil Aristoxenus (4th century BC), who wrote on the theory of rhythm.[8]
Arsis ("raising") and thesis ("putting down or placing") originally seem to have meant the raising and lowering of the foot in marching or dancing. A Greek musicologist, Bacchius or Baccheios (c. 4th century AD),[9] states: "What do we mean by arsis? When our foot is in the air, when we are about to take a step. And by thesis? When it is on the ground."[10] Aristides Quintilianus similarly writes: "Arsis is the upwards motion of a part of the body, while thesis is the downwards motion of the same part."[11] And in general Aristotle (4th century BC) wrote: "All walking (poreia) consists of arsis and thesis."[12]
Because of the association between rhythm and stepping, the parts of a rhythmic sequence were referred to as "feet". Aristides Quintilianus (3rd or 4th century AD) writes: "A foot is part of an entire rhythm from which we recognise the whole. It has two parts: arsis and thesis."[13]
Aristoxenus appears to be the first writer in whose surviving work the word arsis is used specifically in connection with rhythm. Instead of thesis, he uses the word basis ("step"). However, in other Greek writers from Plato onwards, the word basis referred to the whole foot (i.e. the sequence of arsis and thesis).[14]
More frequently Aristoxenus refers to arsis and thesis respectively as the "up time" (ὁ ἄνω χρόνος, ho ánō khrónos) and the "down time" (ὁ κάτω χρόνος, ho kátō khrónos), or simply the "up" (τὸ ἄνω, tò ánō) and the "down" (τὸ κάτω, tò kátō).[15] The division of feet into "up" and "down" seems to go back at least as far as the 5th-century Damon of Athens, teacher of Pericles.[16]
Stefan Hagel writes: "Although the significance of the ancient conception [of upbeat and downbeat] and the applicability of the modern terms are disputed, there is no doubt that arsis and thesis refer to some type of accentuation actually felt by the ancients. Especially in instrumental music, this must have included a dynamic element, so that it makes good sense to transcribe the larger rhythmical units by means of modern bars."https://en.wikipedia.org/wiki/Arsis_and_thesis
