Abraham Maslow (1908-1970) was a psychologist who tries to find a systematic answer to subsistence question, which is known as the Hierarchy of Needs Theory. He said the key to all human activity is the desire to satisfy needs that always arise and emerge. In the hierarchy of human needs theory consists of five layers of vertical levels

1. Physiological Needs (Physiological Needs)
The most basic needs, such as: food, clothing, shelter, breathing, defecation, urination, etc...

Erik Homburger Erikson was born in Frankfurt, Germany, in 1902. His father was a descendant of Denmark and His mother was a Jew. Erikson studied psychology at the Anna Freud (daughter of Sigmund Freud) in Vienna Psycholoanalytic Institute during the period 1927-1933. In 1933, Erikson moved to Denmark and there he set up psychoanalysis training center. In 1939 he moved to the United States and become citizens of the country, where he had taught at several renowned universities such as Harvard, Yale, and University of California at Berkley.

Some of the books ever written by Erikson and extraordinary acclaim from the public include: (1) Young Man Luther: A Study in Psychoanalysis and History (1958), (2) Insight and Responsibility (1964), and Identity: Youth and Crisis (1968).

Sigmund Freud was born on May 6, 1856 in Freiberg (Austria), during the rise of Hitler, and died in London on September 23, 1939. He was a German Jewish descent. At the age of 4 years he and his family moved to Vienna, where he spent most of his life. Although his family was Jewish but considers that Freud himself was an atheist.

In 1900, Freud published a book that became a milestone birth of psychoanalysis psychology stream. The book entitled Interpretation of Dreams which is still known until this day. In this book Freud introduced the concept called "unconscious mind" (natural unconsciousness). During the period 1901-1905 he published several books, three of which are The psychopathology of Everyday Life (1901), Three Essays on Sexuality (1905), and Jokes and Their Relation to the Unconscious (1905).

Ivan Petrovich Pavlov was born in Rjasan on September 18, 1849 and died in Leningrad on February 27, 1936. He is not really a degree in psychology and did not want to be called as a psychologist, because he was a fanatic undergraduate physiology. Pavlov's famous experiments in psychology began when he conducted studies of digestion. In that study he saw that his research subjects (a dog) will salivate in response to the emergence of food. He then explores the phenomenon and then develop a study of the behavior (behavioral study) is conditioned, known as the theory of Classical Conditioning. According to this theory, when food (food is referred to as the unconditioned or unlearned stimulus - the stimulus that is not conditioned or not learned) coupled or included with the sound of the bell (bell is referred to as the conditioned or learned stimuli - stimuli that are conditioned or learned), then the sound bell would produce the same response, namely the release of saliva from the dog experiment. His work even took to get Laureates Nobel. Besides that this theory is the basis for the development of behaviorism psychological genre, as well as laying the groundwork for research on learning and the development of theories about learning.