Final answer:
The founder of modern psychology was Sigmund Freud, who also influenced existentialist philosophy indirectly through his psychoanalytical theories. While Freud's focus was on the unconscious, sexuality, and early childhood, Carl Jung and Jean-Paul Sartre were more directly associated with existentialist thought.
Step-by-step explanation:
The founder of modern psychology, who also had an influence on existentialist philosophy, was Sigmund Freud (1856 - 1939). Freud was an Austrian neurologist who developed the field of psychoanalysis. Through his work with patients experiencing hysteria and neurosis, Freud proposed that the unconscious mind plays a significant role in human behavior. His belief that the unconscious contains feelings and urges of which we are not aware led to techniques such as dream analysis and free association to unlock these unconscious thoughts.
Freud introduced the concept of the psyche being divided into three parts: the id, ego, and superego. His ideas also emphasized the importance of early childhood experiences in shaping an individual's personality. Freud's theories have been widely influential, although his focus on sexual and aggressive drives and the psychosexual stages of development have been met with criticism in modern times.
Notably, Carl Jung, one of Freud's contemporaries and a former collaborator, branched off to establish analytical psychology, discussing concepts such as the collective unconscious and archetypes. Jung's work had an impactful relationship with Abstract Expressionist art movements and is associated more directly with existentialist philosophy than Freud, resonating with European intellectuals like Martin Heidegger and Jean-Paul Sartre.