In reading my new book 'Complete Classical Music Guide', I understand the following: 

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> If two notes are separated by a consonant the sound is pleasing to the ears. If they are of a dissonant interval they clash with each other. Therefore they have to be resolved by a suitable consonance. 
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However, I do not understand (as it does not explain): 

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> Intervals considered dissonant have changed since the Middle Ages
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Is this because: 

* we have later accepted that these “sounds” are not correct, as they were originally listed because ...

*or*

* it is wholly opinion at the forefront of why intervals were once considered dissonant?

I would be most grateful if someone could explain why dissonant intervals have changed as so?

Changed in *what* way?

[![complete classical music guide book dissonant intervals](https://i.stack.imgur.com/iwm8X.jpg)](https://i.stack.imgur.com/iwm8X.jpg)

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EDIT 30/12 

This subject has particularly fascinated me. I have done some further reading. 

I read the following in a PDF article, available [here](http://www.plainsound.org/pdfs/HCD.pdf) *(A History of 'Consonance' and 'Dissonance' by James Tenney)*

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> Musically speaking, there are not really dissonant intervals, but only dissonant notes, Which note is dissonant in an interval physically (acoustically) dissonant. This depends on the clang to which that interval has to be referred By thus distinguishing dissonant.. .notes in place of the old system of intervals and chords, a much clearer view of chords is obtained.
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Whilst an 'Interval' is the difference between two pitches not necessarily a harmony per-se, when these notes are played together they dramatically tell the story of the piece, so I find the above quotation remarkably odd; "there are not really dissonant intervals".

**This** is more interesting from *The Harvard Dictionary of Music by Don Randel*, available [here](https://books.google.co.uk/books?id=02rFSecPhEsC&pg=PA210&lpg=PA210&dq=how%20have%20dissonant%20intervals%20changed%20since%20the%20middle%20ages&source=bl&ots=Um3YRJXpPK&sig=xQTYAIZM2_ebclOH4plX9rDgo1o&hl=en&sa=X&ved=0ahUKEwj-6eWp87LYAhVBKVAKHTEnBO8Q6AEIPTAD#v=onepage&q=how%20have%20dissonant%20intervals%20changed%20since%20the%20middle%20ages&f=false): 

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> During the Middle Ages, theorists generally believed that polyphonic textures were primarily consonant in nature and those imperfect consonances played a subordinate role to perfect consonances. They recognised that phrases begin and end on perfect consonance and that the final perfect consonance is approached by step in contrary motion from the nearest imperfect consonance.
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Are we suggesting then here that this is all perceived by many many factors like temperament, genre, range, and the actual *interval* itself? Almost to a point (as my original post suggests) that opinion is of the highest principle here?

This is what is confusing: An augmented second and a minor third are identical. But in traditional understanding, the first one is considered a dissonance, and the other consonance.

The big question then is can it be argued then that an octave could be dissonant?