Where is the universal principle of verifiability codified and defined?
unfortunate, it is absent.
Even "5 pillars" are not the conventional rule (for example, in the Russian project the fifth item has not been accepted/approved)
On Meta there is only a policy against original researches
See my post about failure of verifiability policy in Russian Wikipedia in wikipedia maillist.
-- Amike kaj kunlabore, Alexander Sigachov http://meta.wikimedia.org/wiki/user:ajvol