[tokai]

Synesthesia is not well understood but it is a falsifiable phenomenon, and the medical consensus is very much that it exists.

[pessimizer]

Counterpoint: it is not a falsifiable phenomenon, and there is no medical consensus that it exists.

[iansteyn]

Source? Or is this satire

[pturing]

If you want to find academic sources arguing against the possibility of measuring and confirming synesthesia or denying its existence, you'll want to dig back to the 1970's. It's not a tenable position in the 21st Century.