Synesthesia (n.) needs no reason: calling a vowel darker, nothing made me use the word, I just used it, with the intonation and gesture I'd use for colours.
Synesthesia (n.) lets some people tell fat days of the week from lean ones, and conceiving a day as fat may consist in nothing but the word together with a gesture of fatness and comfort.
Synesthesia (n.) questions — what colour is the vowel a for you? — can be met with understanding or blankness only by someone who already knows the words.
Synesthesia (n.) like coloured vowels and fat or thin weekdays plays a very subordinate part in our lives, though circumstances could make it matter.
Synesthesia (n.) imagined fully — a sight and a sound felt as the same impression — would we believe the man, and what use could we make of it?