Psychology is a field that traces its roots back thousands of years. In its earliest forms it was a subset of philosophy; Aristotle, Plato, and other Greek philosophers all posited psychological principles. Decartes is sometimes considered the founding father of modern philosophical psychology. He is possibly the most famous dualist; he believed that there was a split between mind and body. The mind influenced the brain through the pineal gland, a small structure in the middle of the brain. It is situated near the ventricles in the brain. Descartes believed that the pineal gland moved the “animal spirits” in the ventricles and sent them throughout the body, through the nerves, to control behavior and movement. 
This was not true but back in the day it seemed a very logical explanation, especially in light of the new discovery about how the heart worked like a pump for blood. The pineal gland was Descartes ideal structure where mind and body interacted because it was in the middle of the brain and was a singular brain structure (most structures in the brain are in both hemispheres. So for example, there are two hippocampi, two caudates, two frontal lobes, etc…). Even with Descartes advances, psychology remained a philosophy until the 1800s when Wundt and other empiricists created experimental psychological laboratories. From there, the field of psychology grew exponentially into the major field it is today.
Many other people influenced psychology over the years, people such as William James, Charles Darwin, B. F. Skinner, Sigmund Freud, and Jean Piaget but Descartes is the earliest of “modern psychologists,” even though he was involved in so much more than psychology. Contrary to conventional wisdom, Descartes was someone who came before the horse and led the way (pun intended; “Don’t put the cart {Descartes} before the horse.”).