Psychology is the study of the behaviors and mental functions of not only humans but animals as well. This study of the human mind can be traced back to ancient Greece, China and Persia, where philosophers wrote out theories on why humans behave in the way that they do. However, the organized study of psychology only began as early as the 1800s and since then several fields have developed that focus on certain aspects of psychology. The most notable of these are clinical, cognitive, comparative, developmental, educational, experimental, forensic, industrial-organizational, school and social. While many of these fields of study often overlap, they each offer a unique and invaluable look at the minds of both humans and animals.