There is nothing one can positively know, except perhaps that one exists, and even that is questionable. Beer kills brain cells, but wine helps your heart. Broccoli is good for fighting cancer, and carrots help your eye sight. Milk builds strong bodies. Eggs are bad.
Salt is bad.
Then science comes along, and suddenly eggs are good for you, salt is good for you, only the right kinds of milk are good for you, and the truth we knew is turned upside down. Trust no one. JFK was not killed by Oswald. Sometimes, Janis Joplin stops by for a drink. The problem with this is that such a personality becomes paranoid, withdrawn, and you end up with lovely people such as the Unibomber, the Branch Davidians, Jim Jones, Heaven's Gate, and the conspiracy-loving Scientologists.