Lets imagine that your vacuum cleaner has stopped working. when it has stopped working in the past, a broken belt was the culprit. since past experience has taught you that the belt is a common issue, you replace the belt again, but this time the vacuum continues to malfunction. you ask a friend to come take a look at the vacuum, and he discovers that one of the hose attachments was not connected, causing the vacuum to lose suction. because of your mental set, you failed to notice a fairly obvious solution to the problem.