The movie "A Knights Tale" a peasant convinces everyone that he is really of noble birth and I a knight so he can compete in jousting competitions so he can make money and become wealthy. So he perceives and believes that he himself is really a knight and changes everyone's perception of him from being a peasant to a knight of noble birth.
In the time that this movie is set your life was believed to be guided by the stars and couldn't be changed so a peasant couldn't become a knight. But the main character "William Thatcher" doesn't believe this and thinks that he can change the stars and become a knight. So his dad gets "William" a job with a knight so he could have a better life that what a peasant would have. While "William" is growing up he learns all the skills of a Knight. Eventually the Knight he is serving dies and he believes he can take his spot convincing everyone that he is his master and a knight changing everyone's perception of him, he does this so he can win and can get some money so they can eat. He does this successfully. Now he has had a taste of competing as a knight he believes himself to be a knight and continues to act so he and the other servants of the dead knight continue on to other tournaments. The other servants continue to train him as a knight. Then they obtain a forged patience of nobility which is needed to enter competitions. But he is known under a new name "Ulrich VonLichtinstien" .
By this everyone's perception of him is of that he is of noble birth and that he is a knight and everyone believes him to be so. So this is their perception of reality. "Williams" perception of reality is that he really is a knight even though he was of a humble origin or in other words he was born a peasant and he also believes that he can change the stars which everyone else thinks in to be impossible to do so. But in the movie he is caught out the he is an imposer and has a chance to escape arrest but because he perceives himself to be a knight he faces it and is an arrested.