Thomas Paine on Ignorance

Ignorance is of a peculiar nature: once dispelled, it is impossible to re-establish it. It is not originally a thing of itself, but is only the absence of knowledge; and though man may be kept ignorant, he cannot be made ignorant. The mind, in discovering truth, acts in the same manner as it acts through the eye in discovering objects; when once any object has been seen, it is impossible to put the mind back in the same condition it was in before it saw it. 

- Thomas Paine ~ Rights of Man