Traditional saying expressing a widely- accepted truth

A traditional saying expressing a widely-accepted truth?

Conventional wisdom?

You could work this one around to mean the above:

SOME PEOPLE DON'T BELIEVE FAT MEAT'S GREASY - Skepticism to the extreme. Doesn't believe what is obvious. Tennessee, African American man in his 20s. Pigeon Forge, Tenn. January 2006.

"Common sense" is a widely used expression meaning "what everyone knows". Pamela