The word [Greek: 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 [Greek: 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 [Greek: 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 ([Greek: poia de poiou biou mimêmata ouk echô legein]), and leaves the matter for future inquiry[1]. [Footnote 1: Plato, _Rep._ p. 400 _b_ [Greek: alla tauta men, ên d' egô, kai meta Damônos bouleusometha, tines te aneleutherias kai hybreôs ê manias kai allês kakias prepousai baseis, kai tinas tois enantiois leipteon rhythmous.]] § 2. _Statement of the question._ What then are the musical forms to which Plato and Aristotle ascribe this remarkable efficacy? And what is the source of their influence on human emotion and character? There are two obvious relations in which the scales employed in any system of music may stand to each other. They may be related as two keys of the same mode in modern music: that is to say, we may have to do with a scale consisting of a fixed succession of intervals, which may vary in pitch--may be 'transposed,' as we say, from one pitch or key to another. Or the scales may differ as the Major mode differs from the Minor, namely in the order in which the intervals follow each other. In modern music we have these two modes, and each of them may be in any one of twelve keys. It is evidently possible, also, that a name such as Dorian or Lydian might denote a particular mode taken in a particular key--that the scale so called should possess a definite pitch as well as a definite series of intervals.