If someone says orbelievesblackiswhite, they say orbelievesomething that iscompletelyuntrue,especiallybecause this ishelpfulto them:
Ministers stillappearon TVtellingviewersthatblackiswhite.
Heaccusedopponentsof "runningacampaignto sayblackiswhiteand up is down."
If youargueblackiswhiteforlongenough,peoplestarttobelieveyou.
Whenpeoplecanpersistentlychoosetobelieveblackiswhite, there's not muchpointindebating.
He isquitestubbornand if hedecidesthatblackiswhite, you willstruggletoconvincehim that he iswrong.