Belief or Knowledge

"It is difficult to get a man to understand something, when his salary depends upon his not understanding it!"
Upton Sinclair.
Everyone is entitled to his own opinion, but not his own facts. 
Senator Daniel Patrick Moynihan.
Since the dawn of civilization philosophers have been struggling with the question "What is knowledge, how do we know?" Sensory perceptions? Cogito erg sum? Works of the sages? Divine revelation? 
Epistemology is a branch of philosophy that seeks to answer this as a "theory of knowledge".
What it all comes down to is that everyones bullshit meter needs constant recalibrating. ASTM Committee F-104 on Bullshit and Irrationality has a protocol for a standard bullshit sample to be placed on a Google search for 'quick money' and the acceptable level for the meter to react in Alarm Mode. A test result of anything over one part per billion needs to have the "?" valve replaced. 
"They say.. everyone knows that... I heard it.. this book.. on TV the ad... well known experts... Rush said on the radio..." Have your meter checked regularly!!