You can use your mental muscles in one of two worlds: the inner world of thoughts and emotions or the outer world of actions and words In the Mental Muscle Diagram (MMD), the inner world is shown to be to the left of the line, whilst the outer world is to right. When drawing a MMD, mental muscles are usually drawn across the line. This is because we use mental muscles, to some extent, in both the inner and outer worlds.

However, Jung observed that people prefer to use them predominantly in one world or the other. That is, when you use the Thinking mental muscle, for example, you usually prefer to use it predominantly either to say or do things, or to develop ideas. Jung would not have expected your preference to be that you use it equally between both worlds. This does not mean that you don't use Thinking equally between both worlds - your circumstances and training may have resulted in this being the case. Jung's view was that your inborn preference was for one or the other - your preference and actual behaviour are two separate things. Bad Joke: If people evolved genetically from apes, why do we still have apes?