John Cotton (also known as Johannes Cotto) was a medieval Music Theorist who lived around 1100 A.D. His treatise, De Musica is important because it provides an exhaustive overview of the state of Music Theory (and practice) from that era. Students of Music History will recall that this era was one of great artistic activity, with the ascendancy of organum and polyphony. In this sense, the treatise summarizes the practice of the ancient past while addressing present and future concerns.
The treatise is largely technical and theoretical, discussing the mathetmatics behind the division of the octave, the history of the modes (scales), and how to write polyphonically. Most of these topics are of little interest to the amateur (or non-) musician, but we do find thought-provoking passages on how Music was once regarded during his era. I'd argue that many of his perspectives are valid today, and that we are all the poorer for it.
For your consideration, I've reproduced some of these quotes below, and added some commentary. 
On the Power of Music
It should not pass unmentioned that chant [music] has great power of stirring the souls of its hearers, in that it delights the ears, uplifts the mind, arouses fighters to warfare, revives the prostrate and despairing, strengthens wayfarers, disarms bandits, assuages the wrathful, gladdens the sorrowful and distressed, pacifies those at strife, dispels idle thoughts, and allays the frenzy of the demented.
Music has different powers according to different modes. Thus, you can by one kind of singing rouse someone to lustfulness and by another kind bring the same man as quickly as possible to repentance and recall him to himself...Since music has such power to affect men's minds, its use in the Holy Church is deservedly approved.
This perspective has been universal in history, and I believe this is still true today. Despite being completely immersed (and thus perhaps, desensitized) to Music, there we still have an unwritten need for appropriate Music. During tender moments in our favorite RomComs, the background music doesn't suddenly switch to gangsta rap. We don't ask for Steve Reich's "Come out" (https://www.youtube.com/watch?v=uGDo1YN_q3c) for weddings. The soundtrack to our gym workouts tends to be of a certain character, etc.
How the Ignorance of Fools Often Corrupts The Chant
We are not sure whether the fact that [errors in singing] these chants...result from the fault of the singers or whether they were thus issued by their composers in the first place. On the other hand, we do know most assuredly that a chant is often distorted by the ignorance of men, so that we could now enumerate many corrupted ones. These were really not produced by the composers originally in the way they are now sung in churches, but wrong pitches, by men who followed the promptings of their own minds, have distorted what was composed correctly and perpetuated what was distorted in an incorrigble tradition, so that by now the worst usage is clung to as authentic.
John pulls no punches on what's permissible in singing. Although the composer may himself be at fault, subjective interpretations in performance can distort an "incorrigible [firmly fixed] tradition," and thus must be identified as erroneous and rejected. He goes a step further and laments the fact that the prevalence of these errors has resulted in listeners mistakenly identifying "the worst usage," as being authentic.
On Errors In Singing
We wish now to consider certain chants that have long since corrupted...and to urge strongly that the corrupted usage be cast out which has debased either these or any other chants whatsovever and has been preserved to this day. Since it is established that the one Lord is pleased by one faith, one baptism, and complete unanimity of morals, who would not believe that he also is offended by the manifold disagreement of singers, who wrangle, not reluctantly or unwittingly, but willfully? Therefore it has not befitted us, who by God's favor have come to know the right way of singing, to tolerate error; nor should we be greatly concerned if certain foolish singers, stubborn in their faults, do not give way to the truth, so long as we reform.
Here's a great contrast drawn between the oneness of God and the diverse, contentious, and intentional corruption of the art by the ignorance of fools. There is a line drawn between people who (by God's favor) are "in the know," with respect to music, and those who "wrangle," (that is to say, create controversy or argue) and stubbornly persist in their errors.
He offers encouragement, however! We should not be concerned or dismayed by these "foolish singers," but rather work toward reforming them in the right way.
How different people have different tastes
It has already been shown by many examples how the modes are differentiated and how they are corrupted by incompetent singers. Yet it should be added concerning their nature that different men are attracted by different modes. Just as not everyone's palate is attracted by the same food, but one man enjoys more pungent dishes, while another prefers milder ones, so assuredly not everyone's ears are pleased by the sound of the same mode.
Rather than being overly prescriptive in his approach, John does allow for subjectivity in how one is affected by the proper use of music. Within that sphere, it is possible for the listener to be affected in different ways. Some might mistakenly interpret this as meaning "all music is relative," but this would contradict the rest of John's treatise. John accomodates individual tastes within the boundaries of what is regarded as desirable in Music. His message is not that "all things are permissible," but rather "within the confines of propriety, there are allowances for personal taste."
- All quotes are from Warren Babb's translation, "Hucbald, Guido, and John On Music: Three Medieval Treatises." (Yale, 1978)