definition of “confirmation bias”

"It is the peculiar and perpetual error of the human understanding to be more moved and excited by affirmatives than by negatives."
--Francis Bacon

Confirmation bias refers to a type of selective thinking whereby one tends to notice, and to look for, what confirms one's beliefs, and to ignore, not look for, or undervalue the relevance of what contradicts one's beliefs.

Example: To argue that smoking is not dangerous by finding seemingly healthy people who smoke is to be guilty of 'confirmation bias' because you ignore the fact that there are unhealthy people who smoke.