[Now then, there being three species into which all harmony is divided, equal in the magnitude of systems or intervals and force of notes and tetrachords, we find that the ancients never disputed about any more than one ; for they never troubled themselves with the chromatic or diatonic, but differed only about the enharmonic ; and there no farther than about the great interval called the diapason. The further subdivision indeed caused some little variance, but they nearly all agreed that harmony itself is but one.*] Therefore he must never think to be a true artist in the understanding and practice of music, who advances no farther than the single knowledge of this or that particular : but it behooves him to trace through all the particular members of it, and so to be master of the whole body, by understanding how to mix and join all the divided members. For he that understands only harmony is confined to a single manner. Wherefore, in short, it is requisite that the sense and understanding concur in judging the parts of music ; and that they should neither be too hasty, like those senses which are rash and forward, nor too slow, like those which are dull and heavy ; though it may happen sometimes, through the inequality of Nature, that the same senses may be too slow and too quick at the same time. Which things are to be avoided by a sense and judgment that would run an equal course.