By Jung's rules, 16 psychological types exist. But in his book "Psychological Types" he described in detail only 8, distinguished by the 8 possible dominant functions. Contrary to Socionics and MBTI, Jung did not conclude that the types had two introverted functions and two extroverted functions. He instead outlined that extroverted personality types had a Dominant extroverted function, with the remaining functions being of varying if lower levels of development that range from being Inferior introverted functions that are necessarily retarded to auxiliary functions that lie in the middle.