In this slide, the first letter of the four-letter code is "I", so the favourite mental muscle is used in the inner world. In the example, the favourite mental muscle is F, but it could be T or S or N.

Bad Joke: An introvert husband and extravert wife were having difficulties in their relationship. One night, the introvert husband went to bed early and wrote a note for his extravert wife saying "please wake me at 5am, as I have to catch a flight". The next morning, he woke up at 9am, and realised he had missed his flight. He was about to get angry with his wife when he noticed a piece of paper by his side of the bed, that said "it's 5am: wake up".