The first letter of your four-letter type code says in which world you like to use your favourite mental muscle.
In this slide, the first letter is "E", which means that the favourite mental muscle is used in the outer world.
In this example, that mental muscle happens to be N - but it could be S or T or F.
Bad Joke: The brain is a wonderful organ. It starts as soon as you get up in the morning and doesn't stop until you get into the office.