[Mark Lapierre]

Some people don't understand how much of an influence their beliefs have over their perceptions. So they don't realise that what they think they perceive is not necessarily what happened. Thus what happened often seems to match their belief, reinforcing that belief.

Others do know this in part, but go in the opposite direction, thinking that what they believe truly affects reality, rather than just their perception of it. The match between their perception of reality, heavily influenced by what they expect to perceive, also reinforces their belief.

In those cases it's incredibly easy to ignore or dismiss or miss anything which disagrees with one's beliefs. Except in extreme situations such as believing you can fly.