Whydo illusions occur?

Illusionsoccur because of a result of a mismatch between the physical stimuliand its perception by the individual. The mismatch is caused byincorrect interpretation of information received by sensory organs.Illusions are called primitive organisations as they are generated byan external stimulus situation that generates the same kind ofexperience in all the individuals. Some illusions are universal innature as they are found in all individuals. They are also known asuniversal illusions or permanent illusions because they do not changewith experience and practice. Contrary to this, illusions that varyin different individuals are known as personal illusions.