Lack of knowledge.
This is a very
powerful force for human good and
evil, and the main relevant difference seems
to be whether one's ignorance is conscious and honestly admitted or else
unconscious or denied.
If one knows one does not know, or knows one
does not know everything there is to know about something, or knows that one
does not know certainly and definitely and with full precision, one can use
his knowledge of one's ignorance to get more and better knowledge.
If one does not know one does not know, or does
not wish to know one does not know,
or pretends to know where one only believes,
one's ignorance is easily dressed up as faith or
ideology, and is often used as a political or
religious power to produce
more ignorance that is dressed up as faith or ideology.
And notice that one may quite certainly know that one is ignorant
about something, or indeed carefully ignore the relevant evidence and believe
one knows something because one does not know and chooses to neglect, dismiss
or avoid whatever is known about it.
"It ain't what a man don't know that makes him a fool, but what he
does know that ain't so."
By contrast, recognized and admitted ignorance about
something is a positive source of and reason for finding positive knowledge
about it, if one can, and not to believe blindly or
wishfully as long as one
doesn't have such knowledge (probably).
And man may be an animal that desires to know, according to
Aristotle, but most men - quite possibly all - actively desire not
to know certain kinds of things, especially such as they disapprove of
or disagree with.