You use all four mental muscles, but you tend to favour two, which are indicated by the middle two letters of your code. Jung observed that survival in the world needs a balance between PERCEPTION (i.e. the S and N mental muscles) and JUDGEMENT (the T and F mental muscles). Therefore, the 2 favourite mental muscles are never both Perception (i.e. S AND N) or Judgement (i.e. T AND F).

Think what would someone would be like if they hardly made any decisions. They might not even make the decision to get out of bed in the morning! Therefore, having SN or TF as your two middle letters is discounted, because Jung observed that people need a balance between perception and judgement. Bad Joke: A factory manager assembled his workforce (of 10 people) and said: "I have some good news - I've got a job to be done that doesn't need any decisions to be made; in fact, the laziest person in the world could do it; so could anyone who is lazy please step forward." 9 of the 10 employees stepped forward. "Why didn't you step forward?" the manager asked, to which the 10th employee replied "I couldn't be bothered".