Each and every one has it in a pair. But nobody is sure if every of them percieve an object to be the same.Take it this way- if I see an object to be red you can not be sure a third person also percieves it as red. I mean he maybe perceiving it as blue but still saying it to be red as per his learnt habit since childhood. So the Biggest problem of the globe today is about belief. A belief which is created in ones mind as the influence of his/her environment.
As you Say:If little knowledge is dangerous then, show me a single man who has as much as to be out of danger.