Jung observed that, in order to achieve a reasonably balanced personality, you usually use one of your favourite mental muscles in the outer world, and the other in the inner world. To put it another way, when you are thinking quietly to yourself, you tend to use one mental muscle, and when you are talking to other people you tend to use another mental muscle.
	In the Mental Muscle Diagram, this is shown by drawing one of the two preferred mental muscles mostly to the right of the line, and the other mostly to the left. Bad Joke: I used to be schizophrenic, but we're alright now.