The word [Greek: tonos] occurs several times in Aristotle with the sense of 'pitch,' but is not applied by him to the keys of music. The nearest approach to such a use may be found in a passage of the _Rhetoric_ (iii. 1, p. 1403 _b_ 27). Speaking of the rise of acting ([Greek: hypokrisis]), which was originally the business of the poet himself, but had grown into a distinct art, capable of theoretical as well as practical treatment, he observes that a similar art might be formed for oratory. 'Such an art would lay down rules directing how to use the voice so as to suit each variety of feeling,--when it should be loud, when low, when intermediate;--and how to use the keys, when the pitch of the voice should be high or low or middle ([Greek: kai pôs tois tonois, oion oxeia kai bareia kai mesê], sc. [Greek: phônê]); and the rhythms, which to use for each case. For there are three things which men study, viz. quantity (_i. e._ loudness of sound), tune, and rhythm ([tria gar esti peri hôn skopousi, tauta d' esti megethos, harmonia, rhythmos]).' The passage is interesting as showing the value which Aristotle set upon pitch as an element of effect. And the use of [Greek: harmonia] in reference to the pitch of the voice, and as virtually equivalent to [Greek: tonos], is especially worthy of note. § 10. _Aristoxenus._ Our next source of information is the technical writer Aristoxenus, a contemporary and pupil of Aristotle. Of his many works on the subject of music three books only have survived, bearing the title [Greek: harmonika otoicheia][1]. In the treatment adopted by Aristoxenus the chapter on keys follows the chapter on 'systems' ([Greek: systêmata]). By a [Greek: systêma] he means a scale consisting of a certain succession of intervals: in other words, a series of notes whose relative pitch is determined. Such a system may vary in absolute pitch, and the [Greek: tonoi] or keys are simply the different degrees of pitch at which a particular system is taken ([Greek: tous tonous eph' ôn tithemena ta systêmata melôdeitai]). When the system and the key are both given it is evident that the whole series of notes is determined. Aristoxenus is the chief authority on the keys of Greek music. In this department he is considered to have done for Greece what Bach's _Wohltemperirtes Clavier_ did for modern Europe. It is true that the scheme of keys which later writers ascribe to him is not given in the _Harmonics_ which we have: but we find there what is in some respects more valuable, namely, a vivid account of the state of things in respect of tonality which he observed in the music of his time.