Ignorant people cannot understand ‘hidden knowledge.’ There are two forms of hidden knowledge. There is the form that one can see in his/her life. For example a disease happens. It appears to come to an individual but it is actually a blessing to help that person reach heaven. This is a hidden knowledge. The second form is based on inspiration. That is a knowledge that belongs to Khidr, angels and prophets. This is not given to people. The fact that the Mahdi is hidden behind veils is also a hidden knowledge.