How do we know anything is 'true'? Basically we feel something is true after repeated evidence of such a thing. This is why it is easy for humans to believe any kind of truth. For example, many feel it is 'true' that Jesus is the son of God. They just 'know' it. Why do they know it? they have read enough stories and listened to enough people who say so, that they come to accept it as true. They internalise the idea as truth and come to feel it in their bones.