According to the 18th century philosopher, Francis Hutcheson, beauty is that which is uniform, i.e. ‘common’, where as what we perceive as ugly is part of variation from the norm of beauty. Said simply, what is less and less common from the norm takes more work to establish as beautiful.
These arguments were laid out in 1725 in his treatise ‘Inquiry into the Beauty of Our Ideas and Virtue.’ They influenced the likes of Benjamin Franklin and Thomas Jefferson and shaped the polity of Liberal Democracy. That is, a political principal founded on reanimating the world into objects of potential, gravitating toward the latent and unadulterated ideal by the careful interjections drawing attention to that potential via a limited formal authority.
Now, take into consideration these ideas would be put be to the test in a constitution that founded the birth of a national identity, and lead to both the American and French Revolutions. At the outset of a political experiment, those ideals began to transform the way peoples viewed themselves within a society.
No less did this definition of beauty as uniformity and ugly as variation decant into the arts, namely music… and, at this time, Haydn, Mozart and Beethoven would have been experimenting with form, it’s unity and symmetries, it’s contrast and variations of order and chaos, beauty and ugliness, structurally strong and weak… in essence, Sonata form.
Now think about this again; Beauty is common, simple, natural, understood. Ugliness is defined as variations on beauty. Holding with the ideal of Darwinian cosmology, that which is ugly, when variation ascends to dominance, becomes beautiful. This principle running its logical course, through Beethoven, Through Wagner, Through Schoenberg, reifies the end of tonality, the epoch of the mapping of the chromatic universe. The end of modernism, the inauguration of post-modernism.