Mary Shelley posits the Lockean notion that at first one's natural condition is that
of synaesthesia, an indiscriminate intermingling of sense experience: only when the
intellect begins consciously to analyze sensory data are they broken down according
to the five diverse senses. The physiological basis for this development had been
posited in the mid-eighteenth century by the French materialist philosopher Condillac,
in his Traité des sensations.
Deliberate synaesthesia is an artistic ploy often to be found among the English Romantic
poets, in none more than Percy Bysshe Shelley: see, for instance, his poem of 1818,
"Lines Written among the Euganean Hills," esp. line 285 ff.