What is it that you know to be absolute, irrefutable fact? Do you know that the Earth is round and the stars in the sky are actually in the universe beyond? Do you know that water is wet and whether there is one true religion?
Many people will answer yes to every one of those questions. But do we really know any of those things? Is there such a thing as fact? I think that there is not. All knowledge that we are able to aquire comes from two sources: our senses - which are ruled by our brains, and other people, who are falliable.
Because our senses are interpereted by our brains, we are able to subconciously "set the dice" and creatively tell ourselves the story of what is going on around us. The brain does not recognize it's own malfunctions. A person could not know that they have a mental disorder if there is no one who is telling them that something is wrong or different about them. Colorblind people would not know that they are color blind if there were no comparison to "normal" color viewing people. Blue as I understand it could be Yellow to someone else, and neither of us may be aware of it. It is also impossible for a single human to experience an event in it's entirety. Our attention is limited to one fragment of what is happening, so even if we are at ground zero we cannot possibly know everything which happened.
As far as knowledge from other humans... I shouldn't HAVE to explain this one but I will. Human beings have a reputation for being incapable of relaying information without interjecting opinion, exaggerating, or omitting certain facts. We cannot re-tell a story without it changing. We also are generally unable to see the other side of things, or to see neither "side" and tell only the factual account of events as they happened. Our emotions are permanently entangled with our recollection and memory, so even if we have a pure desire to re-tell something exactly, we may not be able to do so.
Thusly, everything you have ever been told is most likely a lie, to one extent or another, intentional or not. If you think that you or anyone else knows something, anything, to be true, you are wrong. To comprehend and accept this is to take the first step on the Journey.