Some of the more prominent signs of aesthetic philosophy behind Romanticism apparent in John Keats' "Isabella; or, The Pot of Basil" are (1) the preference for individual experience over universal experience; (2) the preference for powerful emotion and feeling; (3) the preference for emotion over reason; (4) and the preference for symbolism and suggestion over clarity of text. The opening lines of Keats' poem show the poem's orientation to the individual over the general and universal:
Fair Isabel, poor simple Isabel!
Lorenzo, a young palmer in Love’s eye!
Lines 3 through 8 begin the journey of overflowing powerful emotion, representing both the subject's emotion and the poet's emotion:
They could not, sure, beneath the same roof sleep
But to each other dream, and nightly weep.
Stanza IV reveals the orientation of the poem toward emotionality over reason:
So spake they to their pillows; but, alas,
Honeyless days and days did he let pass;
Lines 69 and 70 illustrate the reliance on symbol over direct clarity of expression:
So said, his erewhile timid lips grew bold,
And poesied with hers in dewy rhyme:
Further examples of each are apparent throughout but these early ones set up the orientation of the poem.
One of the revolutionary accomplishments of Romanticism, for better or worse, was to overturn the ancient and enduring presupposition, restated by Elizabethan Philip Sidney, that poetry is a divinely inspired vehicle for imitating the ideals on the heavenly sphere for the instruction of humanity. By emphasizing the individual and individual experience and making the poet the source of poetic inspiration as opposed to a divine poetic inspiration, the Greek and Renaissance philosophy of poetic aesthetic philosophy was reversed. Coupled with this was the idea of the preeminence of the poet, so the voice of the speaker of the poem came to be oftentimes the voice of the poet.
[For more infomation, see "Romanticism" by L. Melani, Brooklyn College, NY from which this answer is drawn.]