De institutione musica I.34: Quid sit musicus.
Now this must be seen: that every art and every discipline has by nature a more honourable principle than the craft that is practised by the hand and work of the craftsman. For it is much greater and richer to know what one does than to do that very thing which one knows; for bodily craft serves as a servant, while reason commands as a mistress. And unless the hand does what reason ordains, it is in vain. How much more illustrious, then, is the science of music in the knowledge of reason than in the work of performing and in action! As much, namely, as the body is surpassed by the mind; for the body, lacking reason, passes its life in service, while the mind commands and leads to what is right. And unless its command is obeyed, the work, lacking reason, will falter. Whence it comes that the speculation of reason has no need of the act of working, but the works of hands are nothing unless they are guided by reason. How great the glory and merit of reason is can be understood from this, that other craftsmen, so to say bodily ones, took their names not from the discipline but rather from the instruments themselves. For the player of the cithara is named from the cithara, the player of the pipe from the pipe, and the rest from the names of their instruments. But he is a musician who, by weighed reason, has taken up the science of singing not in the service of work but in the command of speculation. We see this in the work of buildings and of wars, with the contrary naming of the word. For buildings are inscribed, or triumphs led, with the names of those by whose command and reason they were established, not of those by whose work and service they were completed. There are, then, three kinds that are engaged about the art of music. One kind is that which is performed on instruments; another composes songs; the third judges the work of instruments and the song. But that kind which is set in instruments and spends all its labour there, such as the players of the cithara and those who show their craft on the organ and the other instruments of music, is separated from the understanding of the science of music, since they serve, as was said: they bring nothing of reason, but are without any share in speculation. The second kind of those who practise music is that of the poets, which is borne to song not so much by speculation and reason as by a certain natural instinct. And therefore this kind too must be set apart from music. The third is that which takes up skill in judging, so that it can weigh rhythms and songs and the whole poem. Since this is wholly set in reason and speculation, it will properly be assigned to music; and he is a musician who has the faculty, according to the speculation or reason proposed and fitting to music, of judging of modes and rhythms and the kinds of songs and mixtures and all the things which are to be explained later, and of the poems of the poets.