"The modes of ancient Greek music are of interest to us, not only as the forms under which the Fine Art of Music was developed by a people of extraordinary artistic capability, but also on account of the peculiar ethical influence ascribed to them by the greatest ancient philosophers. It appears from a well-known passage in the Republic of Plato, as well as from many other references, that in ancient Greece there were certain kinds or forms of music, which were known by national or tribal names—Dorian, Ionian, Phrygian, Lydian and the like: that each of these was believed to be capable, not only of expressing particular emotions, but of reacting on the sensibility in such a way as to exercise a powerful and specific influence in the formation of character: and consequently that the choice, among these varieties, of the musical forms to be admitted into the education of the state, was a matter of the most serious practical concern. If on a question of this kind we are inclined to distrust the imaginative temper of Plato we have only to turn to the discussion of the same subject in the Politics of Aristotle, and we shall find the Platonic view criticised in some important details, but treated in the main as being beyond controversy.
The word harmonia, 'harmony,' applied to these forms of music by Plato and Aristotle, means literally 'fitting' or 'adjustment,' hence the 'tuning' of a series of notes on any principle, the formation of a 'scale' or 'gamut.' Other ancient writers use the word tropos, whence the Latin modus and our mood or 'mode,' generally employed in this sense by English scholars. The word 'mode' is open to the objection that in modern music it has a meaning which assumes just what it is our present business to prove or disprove about the 'modes' of Greek music. The word 'harmony,' however, is still more misleading, and on the whole it seems best to abide by the established use of 'mode' as a translation of harmonia, trusting that the context will show when the word has its distinctively modern sense, and when it simply denotes a musical scale of some particular kind.
The rhythm of music is also recognized by both Plato and Aristotle as an important element in its moral value. On this part of the subject, however, we have much less material for a judgement. Plato goes on to the rhythms after he has done with the modes, and lays down the principle that they must not be complex or varied, but must be the rhythms of a sober and brave life. But he confesses that he cannot tell which these are (poia de poiou biou mimêmata ouk echô legein), and leaves the matter for future inquiry."
MONRO, D. B. The Modes of Ancient Greek Music. Oxford: At the Clarendon Press, 1894.